The above link is a link to the videos demonstrating different forms of waves.
Constructive waves are waves that are moving against each other but in the same vertical direction. When they meet the amplitude is combined and once they pass they go back to how they were before they met.
Destructive waves are waves that are moving against each other but in opposite vertical directions. When they meet the amplitude is add together reducing the overall amplitude. Once they pass they return to how they were before they met.