Success Criterion 2.5.7 looks specifically at the activity of dragging an element in order to perform a specific action. An example of dragging movements would be interacting with a digital map where you can use a touch screen to move in a direction to see more of the map outside the available view. Another would be a survey where you are asked to sort a list based on preference where you could move things around by holding down a list item using a mouse and dragging it to the order you prefer.
The dragging motion can be difficult for certain users such as those with difficulty using fine motor skills. What this success criterion asks is that there is an alternative way of performing an action that requires a dragging motion. In the map example, an alternative is to allow the use of a keyboard or swipe (up, down, right and left) in order to move around the map. And when ordering a list, you could look to provide a single pointer method as another way to sort. This is about allowing the user to take step by step actions such as first selecting an item they would like to move by a tap or single click then allow them to swipe up or down to move an item in the direction that is needed. This can also be implemented by other alternative ways such as having a dropdown menu allowing to move the selected item to the desired location.
There is also an exemption in relation to specific cases where dragging movement is needed. An example of this is a game that specifically is about aiming and using the dragging motion is part of the challenge of the game. Or alternatively when drawing in free form on a canvas where the interface is trying to mimic the actual drawing motion.