Minecraft Migrating MC acount?!

So I was just "traveling" on Mojang, and I saw that thing, "Migrate your old Minecraft premium acount into a mojang acount" can someone explain to me what is migrating? I didn't understand nothing from what they wrote...

 

Mmmmm I would help you but I don't know :3

 

It means you use your email address to log in. I did it myself, it's kind of annoying.
I would suggest not doing it XD

 

As far as I know, it's required to use a mojang account now.

It may be annoying, but it's more secure.

For example, if my password was guessed, they'd also have to guess my email address before they have access to my account. (Which is unlikely, as both my email and password are very long. :3)