Bosses are mobs that hurt you and they can and will kill you, but if you beat them the rewards are incredible. Like an example is the Wither, it is hard to beat, but it gives the player a way to get a beacon. This picture is in Lego Minecraft.
The only way to fight the wither is to spawn it. To do that you need to place soul sand and wither sculls in a cross shape. It will fight with the wither effect, it is like poison but will kill you too!
How to reach the Ender Dragon
The strongest boss in Minecraft. Just getting to it is hard because in order to find it you need to go to the nether for ender pearls and blaze rod and craft eyes of ender to locate the stronghold. There is a one in five chance, the pearl breaks, you will need 12 pearls to light the end portal and make it to the ender dragon. Some spaces will already filled with pearls.