It's making snide comments about a problem instead of addressing it directly. I'll give you an example from my life.
A towel in a shared bathroom of my house had fallen off the hook and no one had picked it up. I hadn't even noticed that it had fallen. Instead of picking it up herself or asking the rest of us (there are 4 of us) to be more aware or something, she kept asking, "Does that towel need to be thrown away? It's on the floor and no one has picked it up."
Re: How do you define passive aggressive?
A towel in a shared bathroom of my house had fallen off the hook and no one had picked it up. I hadn't even noticed that it had fallen. Instead of picking it up herself or asking the rest of us (there are 4 of us) to be more aware or something, she kept asking, "Does that towel need to be thrown away? It's on the floor and no one has picked it up."