In a sentence
すみません、excuse meわかりませんdon't understand
Excuse me, I don't understand
How it works
Both words here are fixed polite phrases that happen to share the same negative ending. すみません (excuse me) softens what follows — it's the all-purpose “sorry / excuse me”. Then わかりません (don't understand) is the verb わかる (understand) in its polite negative form: the 〜ます (polite) ending turns negative as 〜ません (not), so it literally means “does not understand”. No subject is spoken — context shows it's “I”.