Boss Update for Minecraft: Pocket Edition adds the Wither, slash commands, Add-Ons, more
The most important change is the introduction of Add-Ons, which allows players to radically alter the way Minecraft feels and plays. For example, with the help of an Add-On, you could change mobs' behavior so you can either ride them or make them explode in different ways.
These Add-Ons can be shared with friends and family over Realms. This is possible thanks to the ability to upload and download your own worlds to Minecraft Realms, a new features that's been added in the latest update.
Furthermore, the Boss Update brings Ocean Monuments, so Minecraft players will now be allowed to explore the depths and their underwater inhabitants, including Guardians and Elder Guardians. Many other water-themed artifact such as sea lanterns, prismarine, and more have been added too.
The Wither is another addition to the game, a powerful mob that Minecraft players have encountered in the PC version of the game.
With the help of slash commands, players will be able to enable various cheats in Minecraft, but that will disable Xbox Live achievements, so you won't be able to obtain some of the hardest achievements using cheats.
Also, a new Creative inventory search feature has been implemented, along with lots of UI and performance improvements. Mojang also mentions that the update includes “ridiculous numbers of bug fixes,” so there's that.