You need to pick up linear algebra and basic probability.
CS229's linear algebra review is pretty good: http://cs229.stanford.edu/section/cs229-linalg.pdf
For more depth, Trefthen and Bau's Numerical Linear Algebra.
For a video course, Gilbert Strang's MIT OCW course on Linear Algebra.
Review of probability: https://see.stanford.edu/materials/aimlcs229/cs229-prob.pdf