Every player has space reserved in the direction of their movement. The size of this space depends on a lot of things (speed, direction of view, playing surface, etc) and is as large as the answer to the question 'if a tree suddenly materialized in this space, could the player avoid contact (without a manoeuvre risking the health of their joints)?'
Moving in a way that this space becomes unreasonably large (running full speed with your eyes closed without checking frequently where you are going would be an extreme example) is considered reckless.
If two players have the same space reserved at the same time and contact occurs, whoever caused the conflict of reservations (i.e. whoever last moved so that their reserved space clashed with the other players reserved space - usually the player who got the reservation last) is guilty of the foul.
Players are free to move any way they like as long as this does not cause an unavoidable collision.
A collision is avoidable for a player if the player could have reacted in time and avoided it, given the circumstances involving their speed and line of sight.