Imagine a typical glyph market on a busy realm: dozens of goblins sitting hunched over their steam calculators surrounding the trading pit, each figuring their costs and profits down to the last copper trying to gain an advantage over the others. Thousands of glyphs are posted every hour, most to be canceled and reposted an hour later at even lower prices. Eventually, one of the goblins has a flash of brilliance (or cracks under the strain; the records aren't clear) and posts all of his glyphs at a loss. The calculation engines grind to a stop, leaving the goblins to stare up at the big board in silence, then at each other. "Now what?"
We call it the wall. One scribe picks a price and tries to hold the entire market to that price. If it holds, the market stops at the wall, and everyone on the other side watches helplessly as sales drop to zero. Sometimes it's done to drive off competitors; sometimes it's done to dissuade new competitors from entering the market, or just to burn up excess ink supplies ... or even just out of boredom to cause pointless drama, goblin style.
Like all good goblin inventions, the wall appears simple on the outside, but remains complicated (and somewhat explosive) when put into practice. One complication is that there is more actually more than one type of wall.