Comment by mindcrime

It depends on how deep you want to go and what your goals are, but I'd say that CuriouslyC pretty much nailed it. Multi-variable calculus, linear algebra, and probability / stats are definitely the core.

If you're interested in finding more "freely available online" maths references, check out:

http://people.math.gatech.edu/~cain/textbooks/onlinebooks.ht...

http://www.openculture.com/free-math-textbooks

https://open.umn.edu/opentextbooks/SearchResults.aspx?subjec...

https://ocw.mit.edu/courses/online-textbooks/#mathematics

https://aimath.org/textbooks/approved-textbooks/

There's also a TON of high-quality maths instructional content on Youtube, Videolectures.net, etc. For example, there's some really good stuff by David McKay (also mentioned in CuriouslyC's post) here:

http://videolectures.net/david_mackay/

Be sure to check out Professor Leonard:

https://www.youtube.com/user/professorleonard57

Gilbert Strang:

https://www.youtube.com/results?search_query=gilbert+strang

and 3blue1brown:

https://www.youtube.com/channel/UCYO_jab_esuFRV4b17AJtAw

as well.