Never heard it said that way before, but I'm guessing they're referring to the fact that the wheel supports weight by standing on the bottom spokes, rather than hanging from the top ones, as you'd intuitively think.

Obviously it's not simply standing on them, since every spoke is in static tension, the bottom spokes just lose some, which has the same effect as compression, but without buckling the spoke.

By far the majority of load is carried by a few spokes at the botom of the wheel, not the full bottom 16 in a 32 spoke wheel.