The woman is covered in 3 different kinds of power series (all Taylor, but one is general and the other two are specific to 1/(1-x) and ln x, respectively) that certain kinds of scientists (presumably personified here by the man) love to swap in for more complicated terms by waving their hands and chanting “first order”. Truncated series give fugly equations a more tractable form by applying certain assumptions (e.g., x is very small, x is very large, or x is fuck-it-we-ball).

EDIT: nvm apparently this is a pop culture reference.

A series of Taylor Series