Reverb is an effect inside of Atom that helps make your sound more interesting. The section is found on the fx section of Atom:

To turn the Reverb section on either turn the "Mix," knob above 0, or click the grey light located on the top right of the reverb section. When the light is pink it means the Reverb section is turned on. You can also turn on/off the reverb section by clicking the light on the reverb module of the fx rack, on the left side of the FX section of ATOM:

The "Mix," control acts like a dry / wet function, meaning the higher the amount the more of the Reverb effect will be applied to your sound. A mix of 0 on the reverb unit will mean no reverb is applied.

The "Size," helps determine how big the reverb sounds. Larger sizes tend to emulate larger rooms or areas, so finding the optimal size value will be up to your personal preference.

The "Decay," knob controls how long the reverb tail lasts for. Higher decays mean the reverb will last for a longer period of time. 

"Lo-Cut," applies a filter that removes lower frequencies in the Reverb. This can help make the reverb sound less dull, and clean up the sound in some cases. The more the knob is turned to the right the more the low frequencies in the sound will be removed. 

"Hi-Cut," applies a filter that removes higher frequencies in the Reverb. This can help make the reverb sound less metallic. The more the knob is turned to the right the more the high frequencies in the sound will be removed.

"Width," makes the reverb signal wider in the stereo field. This can be desirable or undesirable depending on your sound design goals, so be sure to experiment with both wide and narrow settings!
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