A slew limiter takes an input signal and follows it, but at a delayed rate.
Square waves are excellent for showing what a slew limiter does. Visualize a really slow square wave that's two seconds long and 8volts... at the beginning it begins its positive cycle to +8volts and in the middle it dips down to -8volts. It's a nice long square wave.
Now for whatever reason, you don't want an actual square wave. Maybe its abrupt nature doesn't work for your needs. You need something that slopes. Enter slew limiter.
You can set the slew limiter to follow the square wave, but take time to get to the +8volt peak than its natural instant peak. The same can be said for the negative cycle. So the output looks more like a trapezoid than a square.
Slew generators tend to fall into two major categories. You have slew limiters that apply the same slope to both the positive and negative slope. You also have slew limiter that can act independently on the positive and negative slope.
Maths is a popular module that can be used as a dual slew limiter. It has many other functions. Plugging a signal into the input rather than trigger will get Maths to act as a slew limiter.
As far as usage. They can be applied anywhere you want to take a source signal and smooth out any changes in the signal. There's some blur between envelope generator and slew limiters. But an envelope generator is designed take a trigger (square wave) and act in accordance to its duration. If you're dealing with ONE trigger than the functionality will probably overlap. If you're dealing with a series of changes that never return to zero volts. The EG won't track properly.