Snapping refers to the auto-alignment that occurs while moving a layer across your canvas. When scaling a layer, the corners will snap to other layer coordinates (left, center, right, top, middle, bottom) as well as the edges and snap-coordinates of the canvas.
When snapping is enabled, as you move the layer around you'll see it align in relation to the canvas and other layers. The blue lines indicate snapping alignment.
Turning off Snapping:
If you'd rather not use Snapping, you can turn this feature off in the Layer Alignment menu.
On the bottom bar of your canvas, find the 'Alignment' icon to the left. Here you'll see options to have Snapping 'Enabled' or 'Disabled'. You can also use this menu to turn 'Display Distances' on or off.