# What does the notation f|A mean?

Here is an instance of the notation in context.

If $$U_\lambda = F_\lambda(U_0)$$ and $$U_\lambda\cap U_0\neq\emptyset$$, then $$F_\lambda |U_{-\lambda}\cap U_0 :U_{-\lambda}\cap U_0 \rightarrow U_0 \cap U_\lambda$$ is a diffeomorphism and its inverse is $$F_{-\lambda}|U_0 \cap U_\lambda$$.

So what does the notation $$f|A$$ in $$f|A:A\rightarrow B$$ (where f is a function and A and B are sets) mean?