Low grade are things loaded with grains, gluten, by-products. The food is likely very cheap. Friskies, Whiskas, Meow Mix -- Meow Mix is dead last compared to anything else I've looked at anywhere, you can get much better for a cat than it while still spending the same amount. Pretty much most things in a grocery store are in this category. There are a few exceptions; hardly anything comes to mind... but there's Trader Joe's (from Trader Joe's), President's Choice (from any store owned by Jim Pattison; Canadian), 4Health (from the Tractor Supply Company)... all are mid-grade foods.
Mid-grade foods usually have things like several named meats and meat meals in the top 5 ingredients, they may or may not have grains -- they likely will, but will be things like brown rice instead of corn. The food is a step up at least, Kirkland kibble, Chicken Soup, Nutrience and a bunch of other brands in pet stores that I look over for the most part, some pet foods have a high price tag but aren't all that great (for instance, I consider Halo Spot's Stew closer to mid-grade with all the fillers in it, but yet it's more expensive than almost any of the higher quality foods).
Higher quality will be found in a pet store and will likely be quite expensive, and have no grains and hopefully very little to no peas or potatoes. The better foods will have more meat, less fruit and vegetables. There's sort of a split here, low high quality where there's fruit and vegetables and then high, high quality where there's considerably less of everything but meat. The more basic and understandable label, the better. A few brands: EVO, First Mate, Wellness CORE, Wild Calling, Before Grain (which is discontinued but you may still be able to find), Addiction, Tiki Cat, Weruva.