As its name suggests, a dispenser is a mechanism that stores and dispenses items. It can be used to fire arrows,eggs, snowballs, and splash potions at your opponents, and it can hold up to 9 stacks of 64 items.
A dispenser can be crafted from cobblestone, Redstone dust, and a bow. A lever is needed for the dispenser to work, it can be crafted from a stick and cobblestone. You would connect the two using redstone dust. To use the dispenser, you would need to drag the required items into the 9 slots that appear. Now you're all set to use your dispenser.
(To activate pull the lever and the ammunition of your choice will be fired out. Just make sure it's pointed in the right direction.)
