Nether core? Look it up on minecraft wiki! to you mean a nether reactor core? It is the main component in constructing the nether reactor, that creates a 7x17x17 obsidian room, and the core will turn into glowing obsidian that will spawn block and items as well a zombie pigmen! Here is the definition from minecraftwiki.net!
The Nether core can be used in the travel of long distances quickly on the above-ground map. In addition, it also acts as a substitute map in the world of a player, and it can only be accessed from the Nether Portal.