Ores are minerals which can be found underground. When mined, they drop a resource corresponding to the ore; for example, coal ore gives you coal. Sometimes they drop the actual ore itself; which needs to be smelted. With a Pickaxe enchanted with Silk Touch, you can mine all ores so they stay an ore but this decreases the amount of resources you get from Lapis and Redstone ore. Most ores in the game are crucial to survive, such as iron or diamonds to get tools and armor.