We're not perfect, we make mistakes.
It's used to acknowledge that everyone is human and that we all have our flaws. It can be used in a variety of contexts, such as:
* To excuse a mistake or bad behavior: "On n'est pas des anges, j'ai oublié ton anniversaire." (We're not angels, I forgot your birthday.)
* To show understanding for someone's imperfections: "On n'est pas des anges, il arrive de se disputer." (We're not angels, it happens that we argue.)
* To remind someone that everyone makes mistakes: "On n'est pas des anges, tu peux te tromper." (We're not angels, you can make mistakes.)
The expression is often used humorously, but it can also be used in a more serious context. It's a way of reminding ourselves that we are all human, and that we should be forgiving of each other's mistakes.