Best If Used by Date

We have likely all seen the dates stamped on food packages in the United States that say “Best if used by...” or “Sell by ...” or some other phrase that suggests that the food item is not to be consumed after the date specified. It is not really clear by the phrase used if the food item becomes poison after this date, or if the food quality declines after this date, or if the date is just a convenience for the seller to move the product. There is a great deal of confusion about what to do with the product after the date given. Should it be thrown away or can it still be consumed safely but with some degradation of its quality?