Field update action is self-explanatory: it can be used to update a field of the target object of the workflow (and even other related objects, as we'll see shortly).
Let's say that our company mostly sell to Italian customers with an high annual revenue, so if a lead comes from Italy and the web source and the annual revenue is up to $100,000, it should automatically be promoted to a Hot rating. This is a valid configuration for the example field update:
Besides the usual name and unique name system fields, the object type is already filled in (Lead) and the Field to Update box has to be filled in ...