Don't think I've ever heard of anything specifically called a "rescue food", but what I think they're talking about is high carb food that you should always keep in the house in case your kitty's blood glucose goes too low.
One of the favorites around here is Fancy Feast Gravy Lovers foods. The gravy is high carb and if your cat goes too low, you can pop the top, put it back on and "squeeze" the gravy part into a bowl. We usually tell people to start with 1-2 teaspoons of just the gravy (we don't want them eating a lot of the "meat part" because we may want them to be willing to eat later, so we don't want to fill them up)
Another thing to always have in the house is Karo, honey or maple syrup. These are simple sugars that can raise the blood glucose fast, but wear off quickly, so they're good if your cat goes really low to get them back up quickly and then follow with something high carb.