Can anyone tell me what this tool is for?
its called a bevel square.
The term I've hear used is sliding T-bevel. It is used for duplicating angles.
For example, if you are fitting casing to a door with a sloped threshold, you can use the T-bevel to get the angle of the threshold relative to the wall. You can then transfer this angle to the casing or to your miter saw, all without using a protractor.
The link havanagranite provided sums it all.
Is bevel squ.