A half-stop is a position roughly halfway through a slip-joint knife opening or closing where the blade pauses and holds. It comes from a flat ground into the blade tang that the backspring rests against.
The pause makes the knife safer to close. Instead of one continuous snap from open to shut, the blade stops halfway, letting you get your fingers clear before the second half of the movement.
It also stops the blade slamming shut if your grip slips, which on a knife with a strong backspring is a real pinch risk.
Half-stops are a traditional cutlery feature. Their presence, and how crisply the blade sits in the half-stop position, is one of the things collectors judge fit and finish by.
Not every pattern has one, and its absence is not a fault. It is a design choice, and different makers use it on different patterns. See walk and talk for the related quality measure.