Your quiver is a location in your knapsack where you store your projectiles. To set your quiver items, just press shift + q, then specify the letter of your desired thrown or launched weapon or item. Quivering an item (or suitable stack of items) is considered a free action - it will not use up any of your movement turns. This allows you to switch quivered items without penalty to make the most of any bonuses that the quivered item may have, such as switching to blessed arrows to fight demons for the bonus damage.
Quiver is traditionally associated with archery, and the command certainly simplifies the firing of projectiles. However any item can be quivered - useful alternatives could include tripe rations for feeding or taming pets, potions, cockatrice eggs or any rocks or glass in your inventory.
Be wary of firing food to a pet that is eating, as your pet is so intent on devouring the food that the item thrown may hit it, reducing its tameness and possibly turning it from tame to peaceful or even to hostile.
Turning off the warning
The warning "Note: Please use #quit if you wish to exit the game", displayed when using the 'Q' command, can be disabled by setting the suppress_alert option to '3.4.3', or any version number equal to or above 3.3.0.