the short answer
The Bulgarian split squat is a single-leg lift where you rest the top of your rear foot on a bench behind you, then lower straight down on the front leg until the front thigh reaches about parallel, and drive back up. It builds the front-leg quads, glutes, and hip stabilisers in one movement. The single most important cue is to keep your weight in the front leg and treat the rear foot as a kickstand for balance only.
This article is general technique education for coaches and clients, not medical or rehab advice - form is individual, and a qualified coach watching the movement and giving a form check is the gold standard. Clients with pain or a relevant history should clear it with a qualified professional first.