You carefully move 5 feet. Unlike most types of movement, Stepping doesn’t trigger reactions, such as Attacks of Opportunity, that can be triggered by move actions or upon leaving or entering a square. You can’t Step into difficult terrain, and you can’t Step using a Speed other than your land Speed.
DND 5e’s Disengage:
If you take the Disengage action, your Movement doesn’t provoke Opportunity Attacks for the rest of the turn.
My intention would be to create for DND 5e something like:
If you take the Step action, you can move 5 feet to an unoccupied space not in difficult terrain without triggering any opportunity attacks.
As Disengage applies for a creature’s full movement, I think giving the creature an extra 5 feet of movement when taking the Step action makes up for the loss. This would of course apply for NPCs and PCs, so I don’t see this giving an unfair advantage to one group over the other, except for maybe those with the ability to disengage as a bonus action, like Goblins or Rogues; switching that for the Step action about may be a slight nerf to the class/creature/race.
I find the Pathfinder Step makes more sense than the DND Disengage, as I can’t make sense of Disengage working for more than the enemies directly threatening the character: a creature that takes the disengage action could then run there full movement, potentially avoiding a dozen or more attacks.