You already know that 楽器 means "musical instrument." Well, this one has strings, making it a stringed instrument.

This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own. Sure, 楽 has two on'yomi readings, but remember: when it's in a word about 音楽 (music), it's read がく. Just keep in mind that がく shortens to がっ here.