Reserve Voices are part of SynthEdit's Voice-Stealing algorithm.
Synthesisers generally employ a polyphony limit, i.e. a limit on how many notes can play at the same time. This helps restrict the plugin's CPU consumption. Voice-stealing happens when all the synth's voices are in use, but the synth is asked to play an additional note. The synth needs to reassign a voice that is already playing to the new note. There are several different techniques for handing this situation. It's important to understand that the synth can't simply stop playing a voice, because any abrupt stop to a signal causes an unwanted click or glitch.
What SynthEdit does is fade-out the old note, this takes a short time. The disadvantage is that the new note is delayed. i.e. it incurs unwanted latency. This can sound like sloppy playing technique and affect the groove of your music.
SynthEdit offers a fix for this problem, called "Reserve Voices" (aka "benchwarmers") these are simply extra voices that are kept available specifically for voice-stealing. These voices can start immediately on a note-on command. The old voice is still faded-out, but this period overlaps with the new note's attack. Once the old voice is faded-out, it becomes the new Reserve-Voice.
We recommend you choose 3 to 6 Reserve Voices (depending on how many keys you expect to hold down at the same time).