Title. I assume it’s under wind conditions but I can’t find wind conditions
savetheday21 is correct!
A normal map is a bitmap image where each pixel represents the 3 dimensional direction the surface is pointing. Using the angle of the pixel to the light source a program can light/dark shade the pixels on screen to make it look like it is actually 3D. Even though it is only a 2D image.

NORMAL MAP TEXTURE EXAMPLE
So a large section of water could be drawn using only two triangles but look like it is 1000's of triangles. And it will interact with lights in the game in real time. Since the axis we are looking at is always looking back at us, normal maps are usually a blueish color.
There are other types of textures that give similar results these are referred to as Bump Maps or Height Maps.