Symbolism, Imagery, Allegory
You've got a head start on this one if you know what a skeleton key does. A skeleton key gives a person the ability to bypass any lock. It's the ultimate master key.
The irony is that Boori Ma's skeleton keys don't really give her that much access to anything. It should be a sign of her absolute power in the apartment building; she should be able to go in and out of residents' apartments and the building freely or at least be treated as such.
Instead, she's marginalized. She sleeps next to a gate or hangs out on the rooftop or "crouch[es]…in doorways and hallways, and observ[es] gestures and manners in the same way a person tends to watch traffic in a foreign city" (ARD 37). She's just not of a high enough social class for the residents to treat her like she truly belongs in their community.
Which is why it's doubly ironic that those skeleton keys—which she never used inappropriately—end up getting her kicked out of the building.
What does all this mean? Those skeleton keys—symbols of power and accessibility—only become real tools of power and accessibility if Boori Ma is willing to sidestep the rules of her society and break the law.