When you're complicit, you're guilty of being involved in some underhanded or illegal activity. If you see your brother steal the neighbor's cat and don't say anything, you're complicit in a catnapping!
Whenever someone makes it easier for a crime or wrongdoing to be committed, they're complicit in it. A getaway driver may not have actually robbed a bank, but she's still complicit in the crime of robbery because she helped the robber. And you may not be responsible for taking the neighbor's cat, but if you suggested to your brother that nobody would notice if he did (and promised not to tell), you're completely complicit!