A sync tone is a short audio clip that you add to the beginning of a sep (track). It is an artificial sound (snare hit, hand clap, etc.) that is not intended to be part of the finished song. It's a marker. Its purpose is to help you align multiple tracks of a collaboration. If all tracks include the same sync tone, then it makes it easy to align the individual tracks in your DAW, by simply aligning to the sync tones.
Sync tones should have a distinctive, easily-viewable pattern. Multiple sharp (and short) sounds are best, with short periods of silence between the sounds. Three taps on a high-hat, for example, works well. It can be anything. The key is that all tracks must include the sync tone, and all recording must be made relative (in time) to the original track's sync tone.
Below are several popular sync tones. Feel free to download these and use them in your collaborations.