I'm trying to make a slime farm, so I looked at the requirements for a slime to spawn, and it said I needed to find a "Slime Chunk". If anyone knows how to find this, please tell me.
Slimes are bouncy cube-shaped hostile mobs that spawn in the swamp, and occasionally, deep underground.
Slimes spawn in the Overworld in specific "slime chunks" below layer 40 regardless of light levels. They can also spawn in swamp biomes between layers 50 and 70 in light levels of 7 or less."Slime chunks"
Slimes spawn throughout the world (except mushroom islands) below layer 40 regardless of light level, but only in certain chunks, 1 in 10 of all chunks. These "slime chunks" are determined pseudo-randomly by combining their chunk coordinates with the seed of the world.
Source: Minecraft Gamepedia: Slime
What is a chunk?
Chunks are 256 block tall 16 × 16 segments of the Minecraft worlds. Chunks are the method used by the world generator to divide maps into manageable pieces.
Source: Minecraft Gamepedia: Chunk
In short, you can find your slimes underground, but they only spawn in certain parts of the map. To find out were these parts are exactly, you could use a site like this.