Water off a duck's back

A nice idiom  a friend just told me.

Do you know what it means?

Ok...here you have....

If criticism is water off a duck's back to someone, it has

no effect on them at all (often + to ).

Example: She's always being told she's incompetent, but it's just water off a duck's back to her.

Did you get it?
An interesting idiom, isn´t it?