What ghosting is, and why it usually is not a fault
Ghosting is what happens when you hold several keys down at once and one of them quietly stops registering. The name is slightly misleading. The original meaning was a phantom key — a keystroke you never made appearing on screen — but on any keyboard built in the last twenty years the controller prevents that by refusing to report the ambiguous key at all. So what you actually experience is a key going missing, not an extra one appearing. Both behaviours have the same root cause.
That cause is the matrix. A keyboard does not have a wire per key; it has a grid of rows and columns, with a switch at each intersection. The controller scans the grid many times a second, energising one row and reading which columns respond. Two keys pressed together are unambiguous. Three keys forming three corners of a rectangle in the grid are not: current can reach the fourth corner through the other three, and the controller sees a key pressed that nobody touched. Given the choice between reporting a keystroke that did not happen and dropping one that did, every sane firmware drops it.
The fix is a diode at each switch, forcing current one way so the phantom path cannot form. That is what "anti-ghosting" and N-key rollover describe. Cheaper keyboards fit diodes on only part of the matrix — typically the WASD cluster and the modifiers, because that is where gamers notice — which is why a board can be advertised as anti-ghosting and still drop keys in the region nobody thought to protect.
This test and the rollover test answer different questions, and it is worth using both. This page tells you which specific combinations fail on your keyboard, which is what you need if a particular game control keeps dying. The rollover test tells you the maximum number of keys your board reports at once, which is what you need when comparing hardware. A board can have a high ceiling and still ghost badly in one awkward corner of the matrix.