Stick slip refers to the fast movement that occurs between two sides of a fault when the two sides of the fault become unstuck. The rock becomes distorted, or bent, but holds its position until the earthquake occurs. When the rock snaps back into an unstrained position it is called elastic rebound. Stick-slip displacement on a fault radiates energy in the form of seismic waves, creating an earthquake.
When the force trying to make the block (m) slip becomes greater than the frictional force causing it to stick, the block will slip.