This video by 3Blue1Brown provides a really great introduction to the fourier transform in a phenomenal visual way that just makes sense for *why* it is the way it is, which is quite beneficial for any mathematical tool, as they tend to seem very abstract and confusing if you don't have a good understanding of them. This video does a good job of providing this understanding I feel.

Additionally, if you are interested in quantum mechanics, the fourier transform is related to the Heisenberg uncertainty principle and 3Blue1Brown also has a video on it that I would suggest.