In Motion (5e Variant Rule)
From D&D Wiki
If a creature moves at least 10 ft in its turn, but has not used up all of its movement speed at the end of it, that creature may use its reaction to end its turn "in motion". This implies that the creature's movement is continuous from one turn to the next, with all of its actions being done "on the go". The creature gets +1 AC until the start of its next turn for doing this, as they are now a moving target.
A creature cannot delay in their next turn after taking this action.