It's a channel in/out. Their primary function is to provide a place to "insert" a device like a compressor or outboard EQ but can also be utilized as a spot to grab some signal from an individual channel to use as a direct out. Usually the insert is configured right after the trim controll and before the EQ/Fader section of the channel circuit, so when used as a direct out, the EQ and fader have no affect on the signal that you access at that point, only the trim does.