What that sentence says is that, if you call AddInputVector() twice in the same frame, the input vector that the movement component will see when updating the simulation is the sum of those input vectors.
This input vector addition gets cleared when the pawn movement component (subclass) calls ConsumeInputVector()
I don’t think the UPawnMovementComponent actually moves the pawn by itself – I think you’re supposed to override/subclass the component (or add a tick/event handler) that uses the input that’s managed by the component, to actually move the pawn. This is done by the existing subclasses, like Character movement or Floating movement.