Ideally, you want food with one ingredient (e.g. "cherries" or "peas" or "olive oil" or "oregano") and then you assemble it into multi-ingredient food yourself at home (or in the case of the cherries you can eat the one ingredient by itself). If you need to buy multi-ingredient things, then the fewer ingredients they have, the less likely they are to contain weird pseudo-food like coloring agents, the distressingly vague "natural flavors", more preservatives than you really want in your lunch, etc.

This being a heuristic, not a comprehensive meal plan, it has to be simple and easy, so "fewer than five ingredients" is what I said instead of "avoid the following evil food additives". I go into a little more detail in this post of Improv Soup, 2a.