When we set a starting point and a destination point on a nautical GPS, it will show a line that will join the two points and correspond to the ideal route (the shortest one).
In case during navigation we go off course, the STR will indicate the degrees by which we must turn to reach the destination point from this new position.
Normally the nautical GPS shows an "l" (left) or a "r" (right) to indicate if we have to turn left or right.
Before turning it would be appropriate to understand the cause that led us off course. If it is an external cause, such as the current's drift, it is likely that we will have to turn more than suggested by the nautical GPS to attenuate this vector component even in the last part of the journey.