To adress your most basic question,
sidechain is an input on a compressor (or other dynamic processor), that drives peak or rms detector responsible for controlling amount of compression.
Ie, every compressor has a sidechain, just in most of them it is not possible to access it, as it is hardwired to be fed with the same input as audio chain.
Now, in Scope, because of it's modular architecture, it is easy to do sidechaining, and most dynamic devices have such inputs,
but vst specs, while allowing plugins with more than 2 inputs, are not very good about interfacing such plugs with a host.
there are some vst plugins, that can be configured to use left input as audio in, and right as sidechain for example, but it is painfull workaround really. New slew of hosts that allow modular routing, like EnergyXT, Bidule,Audiomulch or Tracktion, and, as mentioned by Valis, new VST3 specifications should change it soon.
as for what sidechain is useful, 2 examples along of what you've asked about:
- if you put a compressor on a bass track and feed it's SC input with kick track, then you can compress bassline when kick crosses some treshold, so bass doesn't compete with kick for sonic space
- if you put a gate/expander on a bass track and feed it's SC input with kick track, then bass will be gated unless kick crosses certain treshold, which will make it very tight with kick. This trick can help with poor bass playing, like mine for example

I hope you've got even more confused
