tunneling is a physical process that can be explained as (not my definition):
"Tunneling is the quantum mechanical process by which a particle can penetrate a classically forbidden region of space (for example, passing from two separate points A and B without passing through intermediate points). The phenomenon is so named because the particle, in traveling from A to B, creates a sort of "tunnel" for itself, bypassing the usual route. "
there are some sites which can explain you better then me.