Movement
This game measures movement on a grid. Difficult terrain and other types of terrain may impede your movement, while elevation is relevant to flying creatures.
Creatures can get tactical advantages by careful positioning. The most common are using cover from terrain and other creatures to increase your AC, and flanking, which requires you and an ally to be on the opposite sides of an enemy to reduce the enemy's AC.
Related: Escape a grab or restraint, falling, forced movement, moving through creatures, special movement modes (burrow, climb, fly, and swim), travel speed outside of encounters, untethered