An insert is a jack on a mixer that allows you to "insert" an effect on a specific channel, sub-group of channels, or main channel. The insert jack is a 1/4" TRS jack in which the Tip acts as the send (or output), the ring acts as the return (or input), and the sleeve is the ground. Using a 1/4" insert cable, you can send the channel's signal to an effect (e.g., compressor, EQ, etc.) and return it to the mixer through the insert jack.

Insert jacks can also be used as two different types of direct outs. If you plug in a 1/4" TS (Tip-Sleeve) cable all the way in (to the second click), the signal is sent out from the tip of the insert jack, but does not return through the ring. This breaks the circuit and interrupts the signal flow through the rest of the channel (referred to as a direct out with signal interruption).

If the 1/4" TS cable is only plugged in to the first click, the signal is sent out the cable, but also continues through the rest of the channel (referred to as direct out without signal interruption).

Inserts can be particularly useful in recording applications, as they allow a direct out for each channel on the mixer.