Captive Minecraft is a pretty old minecraft map made by The Farlanders (http://thefarlanders.com), stuck in the 1.11 update using achievements but today, I bring to you the 1.18 version with advancements!
For a better explanation, given by the official site, I quote it here below.
"Captive Minecraft is a new survival game type that traps you in a world border - an impenetrable wall which prevents you from exploring the world surrounding you.
When you first begin, the space you have available is quite limited - you can’t move further than the block you spawn on. However, for each Minecraft achievement you earn, the world border increases in each direction by one block. As your captive world slowly expands, you gain access to new resources, special areas, and rewards. "