While dogs are technically omnivores, meaning they can get nutrition from both meat and plant sources, their diets should consist primarily of protein - and most dog foods already use protein as their primary nutrient. However, not all proteins are created equal. That's why choosing the right food for your dog means taking a closer look at just where that protein is coming from.
For example, most dog foods contain one or carbohydrate ingredients like wheat, potato, rice, etc. That's because these ingredients also naturally contain protein (though in smaller percentages when compared to meat), plus they are much cheaper than meat, keeping down manufacturing costs while still producing a product which meets minimum nutritional requirements. However, dogs' digestive systems are better able to process meat than carbohydrates, so the more a dog food sources its proteins from meat ingredients vs grains and vegetables, the more nutrition your dog will absorb from their food before it passes out of their body as waste. That's why meat should always be the first ingredient listed on the bag; regulations require dog food manufacturers to list ingredients in order of weight.
That said, if a food lists a meat ingredient first, then a whole bunch of different carbohydrate ingredients, that manufacturer may be trying to pull a fast one, using a higher percentage of non-meat protein sources overall, but splitting them among several source ingredients so they can still show meat first on the label. That's why the best foods won't just show how much protein their food contains, but what percentage of that protein is specifically sourced from meat ingredients.