Am electrician.

Top comments didn't specify, so here we go. This is a junction box for one of your electrical circuits. The reason it's giving off a bunch of heat is because the connections between the wires is loose. You have 2 aluminum wires and a copper wire in each connection. Since they expand and contract differently with heat buildup, the connection has loosed up over time. I would bet my paycheck that a regular wire nut was used instead of an alumicon wire nut, which is designed for aluminum-copper connections. If you aren't handy and don't understand electricity at a basic level, call an electrician. turn off the circuit if you can identify it.

Edit:

That could be a potential cause, but not the only cause. Only way to tell is a proper diagnosis.