along; parallel to; in accordance with
Used after a noun to indicate following a physical path, a plan, a rule, or an expectation. The physical and abstract uses are closely related.
I walked along the river.
Let's proceed in accordance with the plan.
に従って also means 'in accordance with', but often implies following a person's instructions or a natural change, whereas に沿って is more about following a predetermined line, plan, or rule.
The te-form of the verb 沿う (そう, 'to run along, to follow'), used as a compound particle.