Tangential refers to something that's not part of the whole. If you make a comment that is tangential to the story you're telling, it's a digression. The story could still be understood without it.

In geometry, a tangent is a line that touches a curve in one spot but doesn't intersect it anywhere else. Tangential means something that goes off in one direction that way and doesn't return. People can feel tangential––as though they're inessential and not relevant to a larger group.

Definitions of tangential

adj of superficial relevance if any

“a tangential remark”
having no bearing on or connection with the subject at issue

adj of or relating to or acting along or in the direction of a tangent

tangential forces”