What does this mean?
Everyone understand what they are able to understand in mathematics. Not everyone can understand everything in mathematics, but at least they can understand what they can.
Not natural but with surrounding context it could make sense.
Everyone understands what HE can from maths.
that wouldn't really make sense tho
EveryONE (singular) understands what HE/SHE (singular) can from mathematics.
So they as a gender neutral...?