A sack is a bag. In some parts of the country, store clerks put your stuff in a sack, but in other parts the same stuff goes in a bag. Sack is also an exciting verb.
While a sack is often just a bag, as a verb it can do a lot more. A boss might sack, or fire, an employee who’s no longer needed. In American football, if you sack a quarterback, you tackle him. This comes from the other meaning of sack, which is to raid and plunder a town, like when the Visigoths sacked Rome. If someone says they’re going to hit the sack, don’t worry, they’re just going to bed.