Two tones are said to be in unison if they are on the same pitch. Unison can be viewed as the musical interval consisting of zero steps of not just 31-ET, but any scale.
Playing in tune is usually important to most musicians, but when playing in 31-ET, playing unison in tune becomes particularly important for an additional reason.
In 31-ET, because the step size, the diesis, is so narrow, there are times in which that interval can be perceived as an out-of-tune attempt at unison. If an ensemble is sloppy about their intonation when playing notes in unison, listeners will have a tough time distinguishing the diesis from out-of-tune unison notes. If, on the other hand, unison is consistently played with good intonation, it will be easier to hear the diesis as a dissonant interval rather than an out-of-tune unison.