She systematically exfiltrated a lot of information which exposed several lies that the U.S. government was telling its citizens and the rest of the world. This information can be divided into four groups: Cablegate, Guantanamo Bay, the Iraq War, and the Afghan War.
The US government was telling U.S. citizens that the people who were stuck in Guantanamo Bay were very dangerous terrorists who would likely be responsible for the deaths of civilians if they were released, but could not be processed via normal US law for unknown reasons either. In fact they were largely being held not because of the danger they posed to society, but the intelligence that they could provide the US. Collateral damage included two British nationals and a journalist. It also made it clear that this intelligence was largely extracted via torture.
The US government had downplayed civilian deaths in Iraq, arguing that it wasn't a huge issue and that most of our killing was very reliable and well-targeted. The current estimates of civilian deaths are closer to 2:1, that's 2 civilians killed for each insurgent killed. I am not misspeaking when I say that: the current estimate is that the majority of deaths were civilian casualties. (One particular hot-button video showed two front-line reporters with cameras gunned down from a helicopter even after they'd apparently shown themselves to be not a threat; the video however lacks two pieces of context which somewhat ameliorate one's outrage: that one of the people they were with did have a rocket-propelled grenade, and that there was a lot of fighting in that region anyways: it was a preventable overstep rather than the murder that was claimed in the video description.)
The US government in Iraq indicated that they take reasonable precautions to prevent things like torture, prisoner degradation, and other abuse of force; these documents revealed that this was not quite correct: while the US government was certainly making efforts to keep its own nose clean in the wake of the Abu Ghraib scandal, it often turned its back on similar awful practices happening at the behest of both the Iraqi police and the US mercenary corporations that it contracted, allowing them to go on with little objection and little indication to the rest of the world that these things were happening. In fact it was not just tacitly accepted, in many cases it was encouraged: US forces turned over thousands of prisoners to the Iraqi police, even though they knew that this was likely to cause them to be tortured and abused. It was damning that US forces ousted Saddam Hussein nominally for being awful to his own people (after we got into the war because he was nominally pursuing nuclear and biological weapons, which it turned out he wasn't), but then bolstered a follow-up regime which was just as awful to its own people.
A related disclosure of Afghan war documents revealed as I recall both the brutality of the Taliban and also surprised us by telling us that there was a good chance our nominal ally in rooting them out (Pakistan), was colluding with the Taliban behind the scenes. It was also a scandal for Canadians, who told their people that certain Canadian soldiers had died in a firefight with the Taliban when those documents appeared to document their dying because the US had accidentally dropped a bomb on them. For the US it was mostly scandalous because we'd understood that we were winning the war in Afghanistan; the results of these documents were much more bleak.
Finally Cablegate shocked the world because it revealed what US diplomatic informants were really saying. For example, US informants revealed a close connection between Putin and Berlusconi, and that Al Qaeda was getting its funding from Saudi Arabia, and that Qatar was a virtual safe haven for terrorists, and that Bashar al-Assad was systematically telling the US with his right hand that he wasn't arming Hezbollah while arming them with his left, and that the Yemeni government was systematically claiming responsibility for US counterterrorist operations in Yemen so that we wouldn't know that that was a front in the War on Terror. Other things emerged, e.g. the US was trading visits by President Obama for other people taking Guantanamo Bay prisoners off our hands; and King Abdullah of Saudi Arabia was embarrassed because he had privately called the then-heads of Iraq and Pakistan awful people; there were some other embarrassments in Cablegate too.
For her disclosure of these things, Chelsea (who at the time was nominally cisgendered and named Bradley) was detained and treated indecently to widespread criticism, and was finally convicted of every charge but the most severe, being sentenced for the remaining 21 crimes to 35 years in jail. This is despite the fact that her disclosure (in combination with WikiLeaks) was carefully redacted to protect vulnerable people, she won several awards around the globe for revealing the information that she did, and her disclosure ultimately caused the Arab Spring which was generally seen as a good thing for the US and democracy. It is hoped that Obama might commute her sentence at the end of his presidency, but that's a pretty slim hope; he has not publicly expressed any sympathy for her.