There are a few reasons why your grass might be wavy. One possibility is that your lawn has a thatch problem. Thatch is a layer of dead and living grass roots, stems, and other organic matter that builds up on your lawn over time. If the thatch layer is more than half an inch thick, it can start to restrict the flow of water, air, and nutrients to your grass, causing it to become weak and unhealthy. You can remove thatch from your lawn by power raking or verticutting. Another possibility is that your grass is suffering from compaction. Compaction occurs when the soil is too densely packed, preventing air, water, and nutrients from reaching the roots of your grass. You can improve soil compaction by aerating your lawn. Finally, your grass might be wavy because of poor drainage. If water isn't draining properly from your lawn, it can cause the grass to become waterlogged, leading to root rot and other problems. You can improve drainage by installing a drainage system or by improving the grading of your lawn.