Basis can have a few meanings, but they're all related. In a literal sense it means the support or foundation for something, but it can also be used to talk about the way an activity is carried out, or the reasons behind something. To steal a few examples from the dictionary: "Trust is the only basis for a good working relationship." "That argument has no basis in fact." - in the sense of a foundation or support "She needed coaching on a regular basis." "Some markets operate on a cash-only basis" - the way that something is carried out. "On the basis of this data..." - justification or reasoning behind something. Hopefully that was clear!