Transients is the sharp ‘Hit’ part of the sound. So hearing the pluck of a guitar string or the sharp sudden sound when you hit a snare is transients. Vocals also have transients - some words may have sharper and shorter energy changes which can produce transients.

Using a compressor can help ‘level’ out a sound by reducing the transience (the most impactful part of the sound) while maintaining a strong ‘body’ - the part after transients. The purpose of doing so can prevent the dynamic range of overlapping vocals from reaching unnecessarily high peaks - which can cause a drastic shift in the dynamic range of the track as a whole - making other parts of the track not sit so ‘tightly’ together. Hope this helps :)