New Players: Please do yourself a favor. Turn on item descriptions.

The game does NOT make this obvious, but if you're on Rep+ then once you've beaten Mom's Foot at least one time, you can go into your settings and toggle something called "internal item descriptions." This will give you a popup every time you encounter an item you've seen at least once and tell you exactly what it does.

If you prefer more detailed info or not having to see items first, you can go on the workshop and download the original mod version called "external item descriptions."

Please my brothers in Isaac be kind to yourselves. There's like 1,300 items in this game.