Slow flight is described as flight at the minimum controllable airspeed of an aircraft. In the ASK21 we train in, depending on various factors, this will be around 40 knots.
So what happens if we try to go slower than "slow" flight? You will stall the aircraft. Now, that's not the terribly scary-sounding word you hear about aircraft in the media. It simply means that one or more of the airfoils on the aircraft stops producing lift (or produces less of it).
More technically, a stall occurs when an airfoil exceeds its "critical angle of attack". The angle of attack is the angle between the chord line of the wing and the relative wind. See below.
Of course, we also need to be aware that the stall speed of a glider is a function of the load factor on the aircraft. The higher the load factor, the higher the stall speed.
When does load factor increase? When you pull back on the stick, and/or when you turn. See below for stall speeds of a glider in various angle of bank: