Anisotropic filtering is texture filtering that processes textures directionally. To keep the computational effort for high- resolution textures as low as possible, one implements the properties of human vision into the texture filtering. In reality, this means that high-resolution textures have higher detail loss and become increasingly blurred with distance. The transitions become smoother corresponding to human visual perception. Due to the loss of detail, however, less computing power is needed to render the textures. In 3D renderings that work without anisotropic texture filters, you can see the jumpy transitions in the displayed textures.
In addition to anisotropic filtering, there is also bilinear filtering and trilinear filtering.