The unreasonable power of non-uniformity
A flip answer is that this isn't the first thing about complexity theory that I'd try to explain to a layperson!  To even appreciate the idea of nonuniformity, and how it differs from nondeterminism, you need to be further down in the weeds with the definitions of complexity classes than many people are willing to get.

Having said that, one perspective that I've found helpful, when explaining P/poly to undergraduates, is that nonuniformity really means you can have an infinite sequence of better and better algorithms, as you go to larger and larger input lengths.  In practice, for example, we know that the na&iuml;ve matrix multiplication algorithm works best for matrices up to size 100x100 or so, and then at some point Strassen multiplication becomes better, and then the more recent algorithms only become better for astronomically-large matrices that would never arise in practice.  So, what if you had the magical ability to zero in on the best algorithm for whatever range of n's you happened to be working with?

Sure, that would be a weird ability, and all things considered, probably not as useful as the ability to solve NP-complete problems in polynomial time.  But strictly speaking, it would be an incomparable ability: it's not one that you would get automatically even if P=NP.  Indeed, you can even construct contrived examples of uncomputable problems (e.g., given 0n as input, does the nth Turing machine halt?) that this ability would allow you to solve.  So, that's the power of nonuniformity.

To understand the point of considering this strange power, you'd probably need to say something about the quest to prove circuit lower bounds, and the fact that, from the standpoint of many of our lower bound techniques, it's uniformity that seems like a weird extra condition that we almost never need.