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

Another neat feature of superposition can 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.