The grid refers to the grid on the map they are using. It is a way to tell the crews how to find the address quickly on their maps. You have probably seen an old fashioined paper map book (like Rand McNally), each page is numbered, and on each page there is a "grid" along the top and along the side of the page. You look up the name of the street in an index, it it will tell you which page and grid on the map where the street is shown.
The grid will vary depending on the brand name of the map, so unless you have the same map, the grids don't mean anything very useful.
If the agency uses commercial maps, like Rand McNally, you can buy one and follow along. But I think this day and age, most emergency services use their own mapping, so they can include detail like fire hydrant location not otherwise available. Also, some departments have their maps online as a sort of public record, ususally for property appraisal records, but sometimes these are "universal" for all agencies in the county to use the same map references.
Finally, years ago I contacted a Orange County florida fire department to find out if I could buy a copy their "custom" map book-- the map books were updated couple of times a year as streets were added etc, and the fellow in charge was very helpful. He took my name, and the next time they updated and distributed "new" map books to crews, he simply gave me an old book that would have otherwise been turned in and trashed. So if you want to pursue it, that's a nice approach.
