I only just update it today but I am confuse how it work.
It say I can legendary my one-handed and it will set it to 15 return its perks.
What does it mean?

It means just that: it resets the skill from 100 to 15, refunding all the perks points you invested into it. Therefore, you can level up one-handed again, and continue leveling your character without having to switch to a different play-style due to maxed skills.

I haven't tried Legendary difficulty yet...

thanks, I got kill 3 times by a legendary dragon.
New tactic must come in.

Note that is you reset your primary combat skill the game will be pretty hard
If you reset an crafting skill or an magic skill who is easy to level up it will have the same effect.