A partner in crime is a person who regularly helps someone else to plan a crime. A bank robber might tell her partner in crime to wait outside in the getaway car.
The phrase partner in crime means accomplice — anyone who assists with the plotting or actual committing of a criminal act. You'll often find partner in crime used in a less serious way, though: "Okay, two piece of pie are missing. Who's your partner in crime?" If a friend collaborates with you on your practical jokes (or even just hangs around with you a lot), she's your partner in crime.