A stall strip is a triangular shaped piece that is added along the leading edge on the INBOARD 1/3 portion of the wing.
On some airfoils with no washout whatsoever and/or on critical airfoils, the wing can stall all at once which leads to a sudden (and undetectable by the pilot) snap roll that can destroy the aircraft.
By adding stall strips, the inboard portion will begin to stall BEFORE the outboard wing which gives the pilot some input where the planes gets a little mushy WHILE HE STILL HAS CONTROL of the aircraft. When you feel (see) the mushy, you have time to drop the nose and keep flying rather than to have to build a new aircraft.

MOST aircraft of all types use washout which serves the same purpose.
Here's a clearer picture of a stall strip.

Highflight