I’m not sure exactly what they are referring to, but here’s the description from that page you linked to:
In the middle ages, practitioners of the art of memory largely abandoned the use of architectural spaces – it had been a fashion of the ancients – and substituted mental grids, either square, circular, or semicircular on the model of an amphitheater. Like building interiors, a grid of rows/columns consists of regular groups of loci (cells) that can be filled with content. Unlike the various parts of a building, however, there is nothing intrinsically distinct and memorable about cells in a grid, so the key to this system is using a cross-referencing code system.
It has been many years since I read The Art of Memory by Frances Yates, but there is a chapter in there titled “The Memory Theatre of Giulio Camillo” that describes one concept of a memory palace in a artificial theater. Here’s a blog post about it:
Here’s another quote from the page you linked to:
In the grid system, the dimensions or axes of the grid – the rows and columns – are different known series, be they numerical, alphabetical, or whatever. If each cell has a row letter and a column number, that two-character combination becomes a hook for the mental image that goes in the cell. Each mental image simply incorporates the code for that cell.
… And the only limit to the number of dimensions of such a system would how many natural series of things you can use for coding. There’s the alphabet – a series 26 items long that almost everyone knows by heart – and the numbers; even just using the digits 0-9 (or 1-10) plus the alphabet gives you a grid with 260 cells – more than I’ve ever needed (but I’ve never needed to memorize a book). Multiply that by the 12 months of the year and … well, you get the idea.
Based on that, it sounds like the author uses a grid of 26 alphabet peg images for columns (or rows) and images for 0-9 (see the number shape system for an example). 26 letters times 10 letter shapes would give you a 260 location grid that you could overlay on an imaginary amphitheater or building.
So if you used the letters for columns, and they were alphabet animal pegs, like A=alligator, B=bear, C=cat…F=fox…Z=zebra, and number shapes for 0-9. then the location F4 would be “fox” with the image for the number 4 (a sailboat, flag, or whatever). You could have the fact you want to store at that location interact with the fox and the sailboat (or whatever images you use for the pegs).
[Edit: the source mentioned row letters and column numbers. My illustration has row numbers and column letters, but the basic idea is the same.]
Giordano Bruno is mentioned on the page. There is some more information about his systems here:
Someone else here might know more about it. I’m just taking a guess. 