Items are tools that can be used in the game. Items can either be obtained when it spawns in different locations, or be bought from the shop. Certain items can give/evolve an ability, change the appearance of stands, while some can remove these items.
For example, DIO's Diary can be used to obtain the variations of Over Heaven stands, like The World (Over Heaven), while the Requiem Arrow can be used to achieve Requiem stands, like Gold Experience Requiem. Some items can even be used to obtain/buy other items. One feature of this item system is that a player can give another player item(s).
Note: To give items, hold the item you wish to give someone, type !giveitem <playername> in the chat, and send the message. It will disappear from your inventory and got to the person you put in for the player name. You can also just type the first few letters of the player's name, and it should still go to that player, unless someone has a similar user to that player, in that case they might be the receiver instead of who you intended.