When a slinky spring is flicked up and down, it generates a transverse wave.
Here's an explanation along with a simple diagram to illustrate:
A transverse wave is a type of wave where the particles in the medium move perpendicular (at right angles) to the direction the wave is traveling. In the case of the slinky spring, when it's flicked up and down, it creates a series of crests (high points) and troughs (low points) that travel along the length of the spring.
Here's a basic diagram to help visualize this:
| /\ /\ /\ /\ |
| / \ / \ / \ / \ | <- Slinky Spring
| / \ / \ / \ / \ |
| / \/ \/ \/ \ |
| / \|
----------------------------------------
Direction of Wave Travel