What does the expression 'partner in crime' mean?

Question:English is my second language. If someone says 'that is where he/she met his/her partner in crime' what would/could that mean?

It means that they work together to achieve something. It's usually used jokingly, like if you pull a prank on someone or complete a project with assistance from another, you could go, "Oh, she's my partner in crime!"
A criminal partner?
It's basically someone that you maybe do a lot with & stick with, no matter what. Usually a reference for mischief being involved. (not any actual criminal act)

I sometimes use this phrase when speaking of my little girls. They do like getting into mischief together.

**it could refer to a couple, but does not have that strict designation of being a romantically involved relationship.
someone who assists in a plot .
an accomplice
someone who shares the credit with you
your partner in what you did/what you are doing

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