When the displacements of individual waves all go in the same direction at a point, the result is a very large amplitude there. This is known as constructive interference.

This can be seen for two pulses traveling in opposite directions along a string. Both pulses have positive displacements in the y direction. When they come together the result is a large peak.

Another neat feature of superposition can also be seen here - after passing one another the pulses travel as if they had never met the other pulse. This is true for waves in general.

The actual string is shown at the bottom in purple. Pulses moving to the right are shown in red, and pulses moving left are shown in blue. What the actual string does is the superposition of the top two pictures.

When the displacements of individual waves combine so as to cancel one another, the result is a small (or even zero) amplitude. This is known as destructive interference.

Again, look at two pulses traveling in opposite directions along a string. One pulse has a +y displacement, the other -y. When they come together they cancel.

Once again, after they pass the pulses travel as if they had never met one another.