Pressure sensitive adhesive.

The glue material is "tacky" - that's a science word in this context.

Imagine an elastic band, maybe holding up your underwear. You can pull it to deform the shape, but it wants to snap back to the original shape.

The glues have lots of little hairs, sub-microscopic in size. When you push/pull them, the hairs move just a little and get fluid enough to move and flow into tiny little microscopic crevices on material. When you stop applying pressure, the hairs stiffen up and get hard - just like holding onto a cliff with your fingertips.

The amount of pressure required to make the glue into a fluid is one property that gets measured. How strong it is attached to a surface before it detaches is another.

tl;dr it's very much similar to Velcro hook-and-loop material