A jarl is a high-status leader: wealthy, influential, war-capable, and surrounded by obligations. A jarl's power comes from land, ships, warriors, gifts, kinship ties, reputation, and the ability to settle or win disputes.
Jarls are not interchangeable with modern kings. In many Viking-inspired settings, a jarl is a regional power who must keep followers loyal and rivals cautious.
Jarls stand above most free people and far above thralls, but a wise jarl still needs public support. A jarl who ignores custom, cheats allies, or insults respected householders invites feud.