I'll add to what the others have said with an example. The scenario at the start of the article is an example of passive aggression, but it's quite subtle and not explained until later on. Jim has made the effort of cooking pasta for him and Aaron. Carbonara in particular deteriorates quite quickly, so after 10 minutes it won't be very good. We see from the story that Aaron is stuck on the phone with work, and once he finally breaks free, the pasta is spoiled and Jim rather annoyed.

Where's the passive aggression? Later on, the article hints that before this played out, Aaron was annoyed at Jim. He intentionally spoiled the dinner (e.g disrespecting the effort Jim invested) by staying on the phone, but because he pretended to be unable to avoid the call it's passive — he can claim not to have spoiled it intentionally.

In contrast, it would be regular aggression if he'd thrown the pasta into the sink.

Basically taking some action to indirectly signal dissatisfaction rather than directly confronting it.

This could be many things. As the article says, it could be as simple as procrastination.

It could also be something more insidious, like Bob works late every evening, so Sally forgets to feed Bob's dog in the morning.

You dislike something, disapprove, or disagree, but express that disagreement through something like simply not inviting the person you disagree with to the next meeting rather than telling them you disagree and why.

I think it's like:

Passive: Uh, your code looks great! Aggressive: Your code is garbage! Assertive: Your code is broken. Passive-Aggressive: It's great that you think your garbage code is great!

This page seems good https://www.psychologytoday.com/us/basics/passive-aggression

RTFA?