This particle add emphasis to the word it's attached to and means "also", "even", "indeed", or "too". Like 만, it can be attached to almost any word in a sentence.

- can be used with other particles except 이/가, 를/을

Examples:

시험도 봤다./I took the test too.
얼마 먹지도 못했습니다./I didn't even eat that much
술을 많이도 먹는데요./You do indeed drink a lot of alcohol!