They are educated guesses, which have meaning. Requiring both is actually really good because it lets the buyer know how much time is between crispiest and best texture vs a conservative safety estimate. Right now only voluntarily using one or the other lets them scare people with a best by date that people read as a safety date.
Obviously people should still check for mold and spoilage because no date will ever be perfect due to the wide possibilities of how the food is transported and stored, but knowing whether stale chips are also likely to be unsafe to eat would be good to know.
"Use by" dates are based on a conservative estimate of when it is likely to be spoiled.
Sell by: when it should leave the store so the customer has a couple of says to eat it. Mostly used here on refridgerated meat and seafood.
Use by/fresh befor/some others: date used for the food most likely being the right texture/flavor/etc.
Best by: extremely unlikely to spoil before this date , although the further off the date is the more likely that it will be safe to eat well beyond this date. For some canned and frozen foods this is likely to be based on complete failure of texture or taste since properly canned and frozen food is safe indefinitely as long as those conditions are maintained.
California us codifying those last two into the packaging so you don't get only one or the other and clarifying what they are supposed to mean. There is not a more precise method that can be done, although the names are a bit too close to each other.