When someone says CSI division or CSI code, they mean a MasterFormat number. The system sorts all construction work into divisions and sections.
Example: a general contractor invites bids by CSI division. A door hardware supplier looks for Division 08, Openings. A gym equipment supplier looks for Division 11, Equipment.
Learn the divisions your trade lives in, and the nearby ones that hold related work. Use the numbers in your proposal so the general contractor can match your price to the spec. Check the table of contents of each Project Manual, because designers sometimes place work in unusual sections.