A Fellowship in Sword, Axe, Spear, & Shield is a sort of group type. During the first session, the group decides what kind of Fellowship they've formed, which grants the entire group certain bonuses, and defines one of the ways they can earn Renown.
Here's a quick rundown of the different kinds of Fellowship and how they earn Renown. The (currently) full writeup for Huskarls is attached.
A fellowship of Celebrants are dedicated to one or more of the pagan gods and earn Renown by protecting sacred places and objects, leading religious rituals, and otherwise protecting their faith from extinction.
A fellowship of Huskarls serves a Nord lord of some importance, usually a jarl or king, and earns Renown when they improve their lord's prestige and act against their lord's enemies.
A fellowship of Settlers is made up of explorers, farmers, and (self-)exiled political dissidents. They earn Renown by securing resources for their community, discovering new landmarks and dangers, and resisting the influence of outside forces.
A fellowship of Vikings is composed of seafaring merchants and adventurers who earn Renown by seizing fast sums of silver, embroiling themselves in the affairs of foreigners, and increasing their own fame and prestige.
A fellowship of Vitkar (Sorcerers) is dedicated to preserving magical knowledge from humans who would destroy it, and preserving human lives from the depredations of the Faen and other supernatural forces. Vitkar earn Renown when they uncover mystical secrets, thwart the machinations of the Faen, and when they influence mortal politics with their magical arts.
A fellowship of Zealots are a band of fanatic converts to the Cult of the Deiarch. They earn Renown by destroying pagan monuments, establishing kirks and congregations, and converting pagans to the worship of the Deiarch.
Let me know if you have any questions about Fellowships, or any other part of the game!