What did John mean in Mathew 3:10 the axe is laid on to the root of trees?

Mathew 3:10 And now also the axe is laid unto the root of the trees: therefore every tree which bringeth not forth good fruit is hewn down, and cast into the fire.

What was John referring to as the root of the trees? It seems to be a single root but out of it are many trees. How do you understand this verse?