Visco: resistant to change of shape.
Elastic: able to return to its original shape after being forced to change.

Imagine lying on a surface that senses exactly how far to let you "sink in" so that every point on the contour of your body is supported.  

When you are not on that surface, it then "flows" back to its original, flat shape. This phenomenon is similar to pushing your hand into the surface of a bowl of water and feeling the water flow to fill every contour and curve of your hand, then return to its original shape once your hand is removed.