The Voicing section determines how many voices Atom uses, and also offers some good controls for glide. It can be found in the bottom lefthand side of Atom:

The "Poly," section controls how many voices Atom uses when making a sound. A poly of 1 means the sound is mono (it can only play 1 note at a time), but higher poly values allow for multiple notes to be played in succession. 

To raise or lower the number of voices click the grey poly box and drag it up or down to increase or decrease the number of voices.

The "Glide," control determines how quickly Atom changes from one pitch to another. Turning the glide knob above 0 will make pitch changes take longer, which can be useful for certain types of sounds.

Turning (or clicking) the "Mono," switch up (and highlighted pink) will force the sound to go mono, meaning only one note at a time can be played. If multiple notes are played at once then only one will be audible. 

Turning (or clicking) the "Legato," switch up (and highlighted pink) will cause the Glide amount to kick in whenever pitches overlap. This can be useful if you want glide on certain parts of a sound for example, and not every single transition from pitch to another pitch.