A person who plays the violin. That is the whole meaning, and it covers everyone from a child in a school orchestra to a soloist on a concert stage — the word says nothing about how good they are.

One useful detail: in an orchestra, the players are usually called violinists, but the leader of the first violin section has a special name in British English — the leader — and in American English the concertmaster. And in folk, country, or Irish music, the same instrument is called a fiddle and the player a fiddler. Same wood, same strings, different world; calling a fiddler a violinist sounds formal, and calling a classical soloist a fiddler sounds cheeky.

The pattern here is worth keeping: instrument + -ist gives you the player. Pianist, guitarist, cellist, flutist. Not all instruments follow it — a person who plays drums is a drummer, not a "drumist" — but for most orchestral instruments it works.