If you didn't have a simulator and were sitting in front of lab equipment, and you wanted to characterize something, you might change a knob over some range and write down the resulting behavior of the circuit, perhaps plotting the data on a graph as you went. The knob could be controlling amplitude, frequency, whatever else you have a knob for on your equipment. You'd be taking measurements with a scope or meter of some kind.
A "sweep" usually means you're only turning one knob, because it's a lot easier to sit there turning one knob a little at a time, pausing for each measurement, than it is to randomly hopscotch over a grid of measurements. If you wanted a wider range of measurements you might do one sweep, change some other settings, and sweep the first one again.
The software is basically doing the same thing for you, to a simulated circuit. Some lab equipment can be programmed to do the same thing to a real circuit. With software, it may be a lot easier to change more than one variable at a time (say both amplitude and frequency), but it can also be more confusing what is affecting what. Holding a system down to where only one variable is changing at a time is a standard way of making it easier to learn what's going on.