NEI

NEI, or Not Enough Items, is a mod made by ChickenBones that is a successor to the popular mods TooManyItems and Recipe Book. NEI modifies the existing Inventory GUI by adding an icon list of items to the right which is searchable via a text box at the bottom of the screen. This mod is in many modpacks as it is useful for checking recipes and uses as well as even searching for items in chests.

Using NEI
In NEI to search items that are in the game you can click the box at the bottom of the screen when you open a GUI (Chest, Workbench, Inventory, etc.). To check how to make a item, search it, then hover over it and click "R" for Recipe; to see what you can make with it press "U" for Utility. To search an item in a chest click the search box twice (it should have a gold outline) and enter the item you want to look for, then open the chest that you want to search in and it will make everything grey except for that item (you can double click the box again to make the search mode go away)