What is the closest star to Earth besides the sun?

Procima Centauri, at about 4.25 light-years away.

Proxima Centauri is believed to be part of a three-star system whose other two stars are collectively known as Alpha Centauri. The Alpha Centauri stars are each about as massive as and orbit each other closely. Proxima Centauri is smaller (a red dwarf) and likely orbits around the Alpha Centauri pair.

https://en.m.wikipedia.org/wiki/Proxima_Centauri