Undulate means:
* To move with a smooth, wavelike motion: This is the most common meaning. Think of a flag waving in the breeze, a snake slithering, or the surface of a pond after a stone is dropped.
* To have a wavy or rippling surface: A hillside can be undulating, or a fabric can have an undulating pattern.
Let me know if you meant a different word, and I'll be happy to explain its meaning!