。
She has a sense of humor.
Literal
[topic-は] humor [subject-が] understands.
わかる ('to understand, be comprehensible') is one of the classic 'subject-with-が' verbs in Japanese — the thing understood is grammatically the subject, marked with が, while the experiencer is the topic with は. So ユーモアがわかる literally 'humor is comprehensible (to her)' = 'she understands humor / has a sense of humor.' This matches the pattern of 好き, 嫌い, できる, ある, いる, and other stative predicates in Japanese.