what si the LCU alpha?

LCU = League Client Update

So yes, you are correct.

Yep it's the new client development

Yup. It stands for League Client Update.

Here's an overview of the features and some announcements/patch notes about it: http://na.leagueoflegends.com/en/client-update

It's a fancy new interface along with lots of updates to the guts of the client to make it more functional and easier to update in the future.

It's the next generation of the Client. Nothing much else to say.