Meaning: The moving ridges of water hit the hard objects made of stone with force.
Choice of Words: Waves are the moving ridges on the surface of the water. Crashed means hit something with force. Rocks are hard objects made of stone.
Alternative Expressions
The waves smashed into the rocks.
Meaning: This means that the waves were hitting the rocks so hard that they were breaking them into smaller pieces.
Choice of Words: The word "smashed" means that the waves hit the rocks with great force, causing them to break into smaller pieces. The word "into" means that the waves moved inside the rocks, causing them to break.
The waves pounded the rocks.
Meaning: This means that the waves were hitting the rocks over and over again with great force.
Choice of Words: The word "pounded" means that the waves hit the rocks repeatedly with great force. The word "the" means that the waves were hitting the rocks specifically.