Nomad

Nomads are visitors to your Village. They come to see if they become a resident.

Spawning
If you have less than two Villagers a pack of Nomads will always spawn.

Otherwise you have a 1/sqrt(villagers) chance to spawn a pack of Nomads. (example: if you have 4 villagers you have a 50% chance, if you have 9 villagers you have a 33% chance that Nomads will spawn)

If a pack of Nomads successfully spawns there will always be at least one. An additional Nomad will spawn if you have less than two Villagers. After that it is an ongoing 50% chance to spawn one additional Nomad.

Use
If you right-click them with a profession token, they will join the village and start working.