So a while back I remember hearing that if you knew a seed to the singleplayer/multipleplayer world you could get glitch items in or something. I have no idea if this is true and it probably isnt. Anyway could someone let me know if this is true or not and possibly tell me how to do it.
Speaking as one with extensive modding/coding experience I can't even imagine how the world seed can have anything to do with glitching items in, and all you need to do to find the seed in singleplayer is use the /seed command, so it is hardly a secret (it is even possible to find the seed for a multiplayer server without commands but doing so requires sophisticated programming knowledge).
Maybe you were thinking of using a tool like AMIDST to locate biomes and structures? This is not "glitching" though, just exploiting the fact that "random" world generation isn't actually random (any seed in the same version will always generate (nearly) the exact same world, down to loot in structures; you could exploit this by selectively deleting the chunks they are in so they will be regenerated; some features, like passive mob variants/stats, are not tied to the world seed so you could regenerate them until you get what you want, e.g. villager trades (only the first trade, the others are always random, as much as the 1.8+ trading system allows).