Imagine how your guitar might sound cranked up in a cathedral, as opposed to how it might sound cranked up in your bathroom. The idea of reverb is recreate that sound. What that means is that we can create a feeling of depth and space for your guitar within a mix or in your band.
Most reverb pedals will feature a level control that effects the volume of your reverb against the sound of your clean guitar, the tone control changes the brightness of the sound and the time control effects the length of the reverb. Some models have an additional control that allows you to line up different characters of reverb.
There are many different types of reverb. Common types include spring reverb that emulates the sound of the reverb units found in many guitar amplifiers. At the other end of the spectrum we have hall reverb that emulates the sound you might hear when playing in an arena. In the middle we have room reverb that emulates the sound of a smaller space, and is great for a bit of rock chuggage.