。
Only mother truly understands me.
Literal
Mother [only-だけ] [subject-が] truly I [obj-を] is-understanding.
だけ ('only') attaches to the noun and is followed by が for a strong 'only X' emphasis — 'it's only mother who understands me.' Notice how だけが feels more exclusive than だけ alone, and focuses on mother as the unique possessor of understanding. ~ている on 理解する gives the resulting-state reading: 'is in the state of understanding.'