Hot mic
With a microphone, hot mic describes when your microphone is always listening (recording) and doesn't require you to press a button. Many games that support voice chat allow users to turn this feature on or off through the game settings.
When the hot mic setting is enabled, everything the player says while playing the game is broadcast to other players. When disabled, a key needs to be pressed to broadcast your voice. The key pressed varies by game and can be adjusted through the key bindings.
If your microphone or headset has a mute button or switch, it can be used when you don't want your voice heard while using a hot mic.
What is a death mic?
In games like Call of Duty and Battle Bit, a death mic or death comms is a setting to turn on the microphone after being killed. This setting lets you say something to the person who killed you.
To not hear other players, games with this feature also allow for it to be enabled and disabled in the game settings. In the game's voice settings look for a "death microphone" setting.