To become assimilated into and accepted by a group. When you do, everything changes. Their marginalization by themselves and by others reflects the economic and societal cost of fitting in. See also: fit fit in somehow with something to match up or harmonize with something in some fashion.
In many respects, conformity or competition is not a choice but simply a matter of survival.
That huge sofa simply doesn't fit in through the door. That dynamic changes when you find people who share a common cause.
In this usage, a noun or pronoun is used between "fit" and "in to. Belonging is incredibly important to healthy human development and the core is about love, acceptance and connection.
It obviously hobbles the wealth of ideas, which in turn limits growth and innovation. People who defy convention are labeled with many disparaging terms: rebel, freak, square peg in a round hole, weirdo, black sheep, oddball.
I just don't fit in with any of the kids at my new school.