This is an OEM Honda engine thermostat gasket – part number 19301-RNA-315 (also 19301RNA315) – made to fit a bunch of Honda models from 2006 to 2015, including the Civic Si, Civic EXC, HR-V, and Acura ILX.
It's a direct replacement cooling system seal that sits between the thermostat housing and the engine block.
If you're doing a thermostat swap or chasing a coolant leak, you absolutely want a fresh gasket to seal things up right.