Decent sounds, bit of a pain to program (if you wanted to) and mini keys but there are some real time controllers for filter and stuff. Cheap (I think) so could be a good starter synth until you wanted better keyboard and/or control?
As above, MIDI is used to control synths and other equipment remotely (note on/off, key transpose, pitch/mod control etc.) In software like Cubase for example, you can record midi data into a sequencer so it plays the same thing on every synth connected to it, or different sequences for different synths on up to 16 different channels. For now, you'll likely just use it to control a synth that does not have it's own keyboard... unless you buy a Microkorg that has its own mini one.
FYI, on original 5 pin din midi connections (that you don't have on that pic) you can daisy chain many synths (out to in and thru to in and so on), so what you play on the master keyboard plays on all the synths connected to it too. There's a lot more to it, but there's the basics.
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