The original joke for anyone who doesn't know or remember, Michael, the manager, asks his accountant to explain what a surplus is like he would to an 8 year old. Michael doesn't understand this and asks for how he would explain it to a 5 year old, which Oscar does a very good job with explaining and Michael finally understands.
Later the security guard is called upstairs to break a tie on votes for what to spend the surplus on, and Michael starts reciting Oscar's 5 year old explanation, and the guard cuts him off saying he knows what a surplus is.
I have to imagine this is sort of a joke about the audience too as inevitably the writers were considering that audience members might not know what a surplus is, and this kind of joke is an opportunity to explain it.
Later the security guard is called upstairs to break a tie on votes for what to spend the surplus on, and Michael starts reciting Oscar's 5 year old explanation, and the guard cuts him off saying he knows what a surplus is.
I have to imagine this is sort of a joke about the audience too as inevitably the writers were considering that audience members might not know what a surplus is, and this kind of joke is an opportunity to explain it.