Send tracks (sometimes simply called Sends) are the software audio routing equivalent to the aux-sends found on multitrack sound mixing/sequencing consoles.
Further, each track can be separately adjusted in many ways, such as changing the volume, adding effects, and so on.
This can be done with individual hardware components, commonly known as "outside the box," or via software applications known as DAWs (Digital Audio Workstations), commonly known as "inside the box."
They are also inherently more flexible than their hardware equivalent since any number of send tracks can be created as needed.
The solutions offered by most multi-track software provide musicians with an easier (although arguably less hands-on) approach to controlling sends and their respective effects on the audio.