$A$ is a set and so is $B$.
$f$ is a function $A \to B$.
I have a math question that asks about $|f(A)|$. What does the notation $|\cdot |$ mean?
If the elements of $ B $ are sets, then the double bars could mean cardinality (i.e. the size of the set).
If $ B $ is a set of numbers, it probably means absolute value.