In simple words-
You can use the yippy illustrations in your personal projects and projects for your clients.
No attribution is required. You can distribute any number of copies of projects created using 'yippy illustrations' (example website designs, app mockups, static or animated graphic designs, etc). 
You can modify the illustrations according to your need.
You can't sell or distribute illustrations from 'yippy illustrations' directly (example selling one or more illustrations as your own illustrations or in the form of illustration packs). 

You're allowed to -
Use the illustrations for commercial and non-commercial purposes without asking for permission from or providing attribution to the author.
Modify, alter or create derivative work for yourself or for a client using the illustrations 

You're not allowed to -
Sell or distribute one or more illustrations(directly or in the form of illustration pack).