Throat thickness is the perpendicular distance from the inside corner of a fillet weld (the root) out to the theoretical straight line connecting the two leg ends. For a 6 mm equal-leg fillet, the throat is 6 / sqrt(2) which is about 4.24 mm.
Throat is the structural dimension that carries the load. The leg length is just what you specify on the drawing because it's easier to measure with a fillet gauge. The shear capacity of a fillet weld is roughly throat x length x weld-metal strength.
For a 4 mm equal-leg fillet on a typical gate hinge plate, the throat is around 2.83 mm and a 50 mm long fillet handles around 25 kN of shear in C350 weld metal. Plenty for a hinge mount on a 100 kg gate.