In terms of sentence structure, "active voice" means the subject of the sentence performs an action; "passive voice," therefore, is somewhat the contrary: the verb acts upon the subject.
See https://www.grammarly.com/blog/active-vs-passive-voice/ for more.
(p.s. I do not work for grammarly and I am not promoting the app)
First active
> I went to the woods because I wished to live deliberately, to front only the essential facts of nature, and see if I could not learn what it had to teach, and not, when I came to die, discover that I had not lived.
Now passive:
> A decision was made to go to the woods because of a desire for a deliberate existence and for exposure to only the essential facts of life, and for possible instruction in its educational elements, and because of a concern that at the time of my death the absence of a meaningful prior experience would be apprehended.
https://theamericanscholar.org/writing-english-as-a-second-l...
However, there are instances when the passive voice is important and should be used. For instance, if you are trying to put an emphasis on the fact that a subject receives an action. This can often be done in a passively voiced sentence where you clarify a certain relationship. In other words, passive voice can be an important part of adding context.
The idea that you should never use passive is largely just people hearing a rule and thinking it is the end all be all. In reality, you're better off learning what both types of structure do, and then choosing one or the other with a specific goal in mind.
Passive sentences are used in blame-deflecting language, such as statements that superficially look like apologies, but don't indicate who is at fault, and so do not indicate any acceptance of responsibility.
1. It lets you know who/what is performing the action, instead of leaving it to the reader to figure it out.
2. It makes the writing less repetitive because each descriptive word becomes a verb, instead of "to be" being in every sentence.
versus:
“The plan’s rates have gone up.”