system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 How someone managed to build a CPU in Minecraft and what it actually does...
gpt
There are a lot of different components to a CPU. One component is an AND gate. An AND gate will take in 2 boolean values (boolean values are either true or false) and spit out true if both values are true and false otherwise. 

On a standard CPU, AND gates are incredibly small, but they don't *have* to be small. Instead of some transistors, we could simulate an AND gate with 3 buckets, some string and a stick. One bucket we place on the stick in a kind of precarious position. The other 2 buckets are placed above the first bucket. There is also a string connected to the top 2 buckets that will (when pulled) tilt their contents into the first bucket. If the first bucket gets enough water in it, then it will tip over spilling the water on ourselves. 

We then ask a question like "Is the left top bucket filled AND the right top bucket filled?", then we pull on the string. If both buckets have water in them, the bottom bucket will tip over and we will get wet. If only 1 or the other bucket is filled, not enough water will make it into the bottom bucket, and we won't get wet. If neither bucket is filled of course nothing will happen.

So we've made an AND gate with cascading water. Again, we didn't have to use water. We could have used any predictable mechanical process.

Redstone in minecraft is a predictable mechanical process. Clever players used redstone to simulate AND gates, as well as all the other components of a computer.

The end result is a CPU. The CPU is much less complicated than the ones in your computer (not to mention much less efficient), but the point of the exercise isn't to make a complicated efficient processor.

As to the "What does it actually do". Generally all CPUs take in 2 numbers and an instruction, and minecraft CPUs are no different. The instruction is usually something like "add these two numbers" or "tell me if these 2 numbers are equal".