You don't really need Geo Nodes for this sort of thing, you can use Displacement. Start with a plane. Subdivide it by 50 or so. Make sure you are using Cycles Engine (for Displacement to work properly).
Add a material to the plane and change the Displacement settings to Displacement Only or Displacement and Bump
Go to the shading tab and use this setup for the basis for the displacement. It's just a noise texture (a procedural artist's best friend) that makes the ground go "up and down" - I reduced the scale and trned down the detail - adjust it to your taste. The MixRGB Node (mixing with white) softens the effect because a noise texture can provide rather strong values for displacement - change the mix to make it stronger/weaker. Shade smooth and you can also add a Subdivision Surface Modifier if you need it smoother (not pictured).
If, however you are intent on using Geo Nodes, you could probably get away with something as simple as this:
If you want something with a more complex, wavy texture (as opposed to just "bumpy"), you could warp the vectors of the noise texture with a wave texture like this:
EDIT - for a more diagonal (and predictable) wave manifestation, try the setup below - I would say it "tiles" better, but that's hard to define because most procedural textures are infinitely tile-able - variances in height and depth can often be solved by setting the mode to 2D (as opposed to default 3D), and using that alongside UV texture coordinates (provided the model is unwrapped sufficiently):
