You can use it for your commercial website. However, if you build a product and want to include (distribute) CKEditor with it then you might need a commercial license.
The condition whether you need the license is based on your product's license. If your product is closed sourced then including an open source product with it without sharing the source code is illegal. So, this commercial/closed distribution license fills the gap. You pay for it and you can distribute your product with the CKEditor closed sourced. Additionally you may do any closed source modifications to CKEditor.
TL;DR: if you want to modify and distribute it with a commercial license then you need to pay, otherwise you are free to use to for free!
