DI box
A small box that turns the unbalanced output of a guitar or keyboard into something a microphone input can take.
Electric guitar pickups have a high output impedance and want to see an input of around a megohm, or the top end disappears. A mic input sits at a few kilohms and expects a balanced signal. The DI (direct injection) box goes in between, lowers the impedance and balances the signal so it can travel down a long stage snake to the desk.
Passive DIs use a transformer and need no power. Active ones have electronics fed by a battery or phantom power, and suit passive basses with a weak output better. Nearly all of them have a ground lift switch, which breaks the earth connection and is the first thing anyone presses when something hums.