What do I do when I find a dimension entrance in Minecraft?
Solution 1:
This is a Stronghold. These structures house, as @James said, an end portal. To use it however, you must first find it!
The Stronghold is a labyrinth within a labyrinth. It is super easy to get lost. Once you finally find it, you'll need to fill the portal's slots with Eyes of Ender. They are crafted by putting an Ender pearl and a piece of Blaze powder together.
Finally, enter in! You will encounter a new enemy I will let you discover for yourself!