Perhaps the easiest way to think of a send is that it's a signal splitter that lets you "send" the track (or bus) not only to its own output but also to somewhere else at the same time.
Back before DAWs when everything was done in hardware studios only had a certain amount of kit. A big studio like Abbey Road might have had a lot of kit, but even EMI didn't have dozens of every bit of hardware.
So let's say you have recorded a band, drums, bass, guitar, vocals back in the 1970s. You decide that some, but not all, tracks would benefit from some reverb but different tracks need different amounts.
You could pay out and obtain as many individual bulky and expensive reverb units as required to put one on the input of every track between tape machine and mixer. Very expensive and a hassle.
Or you could plug a single reverb unit into the mixer so it received input from an auxiliary (aux) bus and sent it's ouputs to mixer inputs. The aux busses being ones that each track could 'send' to using potentiometers in the track channel strip. In other words, they let you split the signal and send it to not only the track output but the input of another bus. If you look at a picture of almost any analogue mixer you'll usually be able to spot aux send pots in the channels.
By setting the reverb 100% wet you could then control how much reverb got applied to each track by adjusting the send controls on each track. The benefit being you only needed one reverb unit and because all tracks went through the same reverb they'd sound like they were in the same acoustic "space".
Nowadays computers are powerful enough to run lots of plugins, a decade ago that wasn't the case so sends were used in DAWs like analogue mixers to reduce cpu load.
So why keep them now? Partly because they're a good way to insert delays and reverbs where you want a similar "feel" on all the relevant tracks. Sends also have creative applications for parallel processing, where you want a signal without a specific effect and the same signal with that effect. They are also very useful, especially if set pre-fader so their volume in unaffected by the track fader, when you need to send several different monitor mixes to different musicians/singers or e.g. give a singer a monitor mix with reverb but can't use track echo because the latency is an issue for the performer.