This spell can convert wildmen to braves of your tribe's color. You should mainly go for the groups of wildmen rather than just one or two wildmen. The convert spell, however, does not work on enemy tribesmen, no matter how hard you try. You need preachers for that job.

You first encounter this spell by worshipping on a Stone Head in level 4: "Combined Forces" and it becomes permanent after worshipping the Vault of Knowledge near your base in level 6: "Building Bridges". You can only have a maximum of 4 charges of this spell.

 

Note that If you play against the AI, their convert range has a larger range and can convert wildmen that the convert doesn't touch.

 

Both in online and offline gaming, you should be only charging convert in the first seconds - or minutes. This allows you to charge fast converts and means you will have a fast population. It's really important to convert as much as you can. In the early stages of the game, even 1 wild is worth a convert most of the times (specially if your population is less than ~20). In later stages of the game, 1 or 2 wildies might not be that worth - not because of the time you need to charge a convert (that should be short) but because you might need to locate your shaman to cast that spell, and time is most of the times more important than 1 or 2 wilds. 

Anyways, even in later stages of the game, if you see more than 2 wildies together and you are in range, converting them is always worth it.

 

Take care when playing some disputive online level like pressure point or face off, you will need to hurry or someone will steal you wildman -  and there are other 3 shamans trying to do that! It's an early skill to be a fast converter, so take that in mind.