Inserts are signal tap points that can be placed anywhere
inside a channel strip. Unlike Auxes, they will interrupt the signal flow,
feeding the signal from before the insert point to its Insert
send(s), and connecting the remainder of the channel strip to the
Insert return(s), both of which are JACK ports which are
visible to other JACK applications.
Inserts are the JACK equivalents of normalized switching jacks on an
analog console.
An insert allows you to either use a special external DSP JACK application that is not available as a plugin, or to splice an external analog piece of gear into your channel strip, such as a vintage compressor, tube equalizer, etc. In the latter case, you would first connect your inserts to a pair of hardware ports, which are in turn connected to the outboard gear.
Inserts will incur an additional JACK period of latency, which can be measured and compensated for during mixing, but not during tracking!