It is the size of the socket wrench.
The difference between a 1/2 size socket wrench and a 3/4 size socket wrench is the difference in size.
The sleeve size is metric. Generally, 1/4 head, 3/8 head, 1/2 head mean that the sleeve is a square mouth. Among them, 1/2 head = 12.5 mm, 3/8 head = 10 mm, and 1/4 head = 6.3 mm.