An Anderson squat is a squat where you start the lift from the bottom from a dead stop.
Some call it a pin squat or bottom up squats.
Typically used by powerlifters to help build strength getting out of the bottom of the squat.
I find Anderson Squats help with hip mobility. For people with tight hips getting into position is a task in of itself.
Because you’re lifting from a dead stop there’s a good transfer to the deadlift.
For more info on equating be sure to check out the Definitive Squat Guide.