A "hold" is a request to be the next patron to check out an item. If the item is currently available, library staff retrieves the item from the general collection and places it on the Hold Shelf (at the Checkout Desk). The item may only be checked out by the requesting patron unless the hold is cancelled.
If the item is currently checked out to another patron, placing a hold puts you "in line" for the item. When the previous patron returns it, Primo will send you an email notice that the item is now available for you to pick up.
Holds do have expirations when the items are returned to general circulation--or transferred to the next holding patron. You can delete any holds you no longer want under My Account→ Requests.