The mob xp farm is built so that you wont have to risk getting killed trying to get a mob drop or xp, instead you can just stand in front of the drop spot for the mobs and hit them once to get xp.

The mob xp farm is a big build and can take some time, but it is a very useful farm to have. If you are in Easy mode mobs wont spawn as frequent, I recommend going into hard mode.

To make the farm as efficient as possible you should light up any caves and tunnels within a 100 block radius of the mob xp farm, so that the mobs will spawn in the mob xp farm rather than in a cave.