It means you can't give the clerk more than the item costs because they won't give you any money back. If it costs $2.25, you must pay $2.25 and not $3.00.

The easiest way to answer your question is by an example. If you buy a loaf of bread and it costs $1.29, that means that you must pay exactly $1.29 with nothing extra (such as $2.00). In other words, the store probably doesn't have change (the 71 cents owed to you).