A wavefront is a line or surface connecting areas in a wave of similar phase. The wavefront is perpendicular to the direction of propagation. When you drop a stone in puddle, the wavefronts are the circular ripples produced. In a three dimensional wave the wavefronts are surfaces, for a point source they would be expanding spheres, for a collimated beam, like a laser, they would be flat planes. The figure below shows a collimated two dimensional wave. The wavefronts, marked W, join the points on the waves that have the same phase. The gap between the wavefronts is the wavelength.
| _ | _ | _ | _
| / \ | / \ | / \ | / \
|/ \ |/ \ |/ \ |/ \
| \ | \ | \ | --> Direction of travel
| \_/| \_/| \_/|
| _ | _ | _ | _
| / \ | / \ | / \ | / \
|/ \ |/ \ |/ \ |/ \
| \ | \ | \ | --> Direction of travel
| \_/| \_/| \_/|
| | | |
W W W W
As can be seen, the wavefronts are a useful physical concept as they represent what we actually see when observing wave phenomena. The above example could be the waves on the ocean as they come ashore.