From the Creative Commons by attribution NoDerivs 3.0: If you remix, transform, or build upon the material, you may not distribute the modified material.

What does distribution of modified material means? Does that mean that if I add any modification to the source material, then the material becomes unusable by license or that it cannot be redistributed as such?
Can I take an icon under this license, add an outline around it and distribute it in with game? Does adding an outline counts as modification of the source material?

https://creativecommons.org/licenses/by-nd/3.0/

From what I've read, the exact details of that clause vary from location to location and even from judge to judge.  There is no bright line that says "this much is a transformative modification, less than that is not." 

Contact the person who created the material to be certain. Most CC authors will give an email answer allowing the use, which generally satisfies the legal written permission requirements.