Teamfight Tactics (TFT) - Items and Unstackable Items

Items can be equipped to Champions to increase their stats and give additional effects.

There are plenty of ways to optain items in Teamfight Tactics. First of all, there is the carousel, where every single champion in the carousel will have one item.
Additionally there are the PvE rounds where you can optain items along with the Mystery Box System.

Champions are only able to hold a maximum of 3 items, but combining two core items will turn into one full item. But make sure that you're building the item you want, because there is no way to split that full item back into its core items again.
However if you put an item on the wrong champion, you can make the items drop by selling the champion.

As of patch 9.18 All items listed above are unique and are not intended to stack.
If one of your champions has one of the unstackable items equipped, applying that same item will cause the item to bounce off.