Tangential describes 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.