A guitar VST is a piece of software that emulates the sound of amplifiers, cabinets, and stompbox effects. It can also be software that emulates someone playing the guitar for you, but these are few and far between, mainly because it’s so difficult to recreate the nuances of a human player.
Guitar VSTs are loaded into your DAW (Digital Audio Workstation) as a track insert, so you can convert your DI guitar tone into a realistic-sounding amplifier with minimal fuss. A lot of guitar VSTs also come with standalone software as well, allowing you to practice and hone in your settings outside of your DAW.