The “missing stair” is a metaphor to describe a certain kind of dysfunction that exists in some social groups. It describes a member of a group who is deeply unpleasant (in the original piece that coined the term, the example given was a rapist, but it was intended to apply more broadly than that). Everyone else in the group treads softly around this person, discreetly warning newbies about them, basically treating the issue as a “missing stair” where, rather than fixing the stair (i.e. kicking out the unpleasant person), they just train themselves to step over the gap, and warn others to step over the gap. If someone steps into the gap (i.e. doesn’t “work around” the problem person and gets hurt by them), that’s seen as the fault of the person who got hurt, rather than the fault of the “missing stair” (the person who hurt them).
The “missing stair” analogy has been used to explain the dysfunction of some groups in geek culture – basically, many groups refused to kick out men who were sexual harassers or open bigots, instead choosing to create a hostile environment for women, people of colour, and LGBTI people. It’s also been used to describe problem employees who keep their jobs despite long histories of sexual harassment (or other horrible flaws but mostly sexual harassment), including in the porn industry (where people who don’t give a fuck about consent are a very big presence).