A crash course was a quick instructional class on the basic details of a discipline.
In 2373, Julian Bashir suggested that if he traded duties with Elim Garak, he would need to be given a crash course in Cardassian field engineering, something the doctor estimated would take five or six weeks. (DS9: "By Inferno's Light")
In 2375, Janeway suggested that rather than The Doctor quickly uploading information on exobiology to facilitate his treatment of B'Elanna Torres, which she characterized as a crash course in the field, they could create a holographic consultant for him instead. (VOY: "Nothing Human")