Thesis theme - what does this mean?

First:

That means if I buy this pack, I will use the theme for any site I have (all my sites), is this right?

Second:

I don't understand what's this.

Please help.

Thanks.

 

Yes.

A client site is owned by someone who you work for. If you are building a site for a client and want to use Thesis, you must buy a "Client Site Option".
If you buy a "10-Site Client Site Option", you can only use Thesis for ten sites, not ten clients.