It's a plot item that is essential to the story, but it's not something you can actually acquire or use in-game. You receive it from the legendary Pokémon Rayquaza, who uses it to help heal Grovyle.
If you are looking for an item with similar effects, you could consider the Miracle Seed, which can be used to restore the HP of your Pokémon to full. You can obtain this item from various locations, including:
* Finding it on the ground in certain areas
* Receiving it as a reward for completing certain quests
* Purchasing it from the Mystery Shop
However, the Miracle Seed is not as powerful as the Amber Tear and does not have the same story significance.