1.31 describe elastic behaviour as the ability of a material to recover its original shape after the forces causing deformation have been removed.

Elastic behaviour is the way that when you stretch an object with this behaviour it will return to its origional shape after the forces stretching it stop stretching it. Eg when you stretch an elastic band and then let go it pings back to regains its original shape and size.
Note that if you stretch an elastic band to far it won't go back? this is because it as reached its elastic limit which, beyond this point, means it looses its elastic behaviour.