This adapter allow you to screw a 2.5" pump union onto a 2" pump thread. In other words, it allows you to effectively increase the size of the male thread on your pump from 78 mm to 94 mm.
If you don't already have them, you may also need to purchase a 2" o-ring for the pump side, and a 2.5" o-ring for the union side.
Inside diameter of the female thread: 75 mm
Outside diameter of the male thread on the pump: 78 mm
Outside diameter of the male thread on the adapter: 94 mm