A follower is someone who worships a god, but is not a member of the priests' guild. If you're not a priest, being a follower is the only way to gain access to rituals.
Pishe does not accept followers, only priests. All other deities do accept followers.
As a follower, you can directly learn two rituals, if you're a high enough level in the skill needed. Each god grants two rituals to followers--these are fixed, so you can't choose them.
Followers can also use rituals from faith rods, giving them more limited access to all rituals.
Followers get automatic access to the "pray" and "meditate" commands
You can only follow one god at a time. Also, wizards and witches cannot become followers.
You can only perform rituals while in alignment (meaning that your god is "very happy with", "pleased with", or at worst "a little concerned about" you). You can check this with "score align" or "score alignment", which indicates how happy (or not) your god is with you.