hi im just wondering what this reservoir is. thank you

 

Coolant overflow?

 

Yep. And in Honda's brilliance, they made it impossible to see the actual level of the coolant in it. I'd suggest moving it it elsewhere in the cabin. I did, because my K20A swap's belt ate right through it, but even for a stock engine, I think it's something that should be fixed if you can/want to.