There are three relative ranges: close, near, and far. Think of these as distances between important places in the battle, not the actual number of units between participants. Changing your range to a target might require a roll as part of the action, such as using strength to climb up a wall, dexterity to squeeze through a narrow opening, intelligence to navigate a hedge maze, or charisma to weave through a crowd.
Close range means that characters can touch one another and engage in melee combat. Everyone in an elevator would be considered close to one another.
Being near others means you can attack with throwing or ranged weapons or approach them with one action, changing your range to close. Everyone in a large tavern would be considered near each other, while two people behind the bar would be close to each other.
Characters that are far away from each other require multiple actions to approach and can only be targeted by ranged weapons. Characters on the other side of a wide river would be considered far from each other, while all the people on one of the shores would be near each other, and the people in a rowboat making its way across would be close to each other.
You can move one range increment as an action and might require a roll, such as climbing a steep hill, jumping between rooftops, or squeezing through a narrow crevice. You can also move and make a melee attack as one action, so long as you don't need to make a roll during your movement.
Having cover gives you advantage on dexterity rolls to avoid ranged or throwing weapon attacks.