It means cool/awesome.
The word "rad" is used by hipsters.

A cheating youtuber/movement player calling himself legit when he fakes his clips wit h cheats

What you say when you learn a new skill just by watching someone else do it.

n: a unit of measurement used to describe a dose of radiation. A single rad is equivalent to 100 ergs per gram or 0.01 joules per kilogram of irradiated substance.

Rapid application development. Used to describe programming languages such as visual basic that speed up development time.

RADness is short for Random Acts of Designess. The definition of a Random Act of Designess is where you surf the web, find a good cause that needs some help with it’s identity, or promotional material (or anything else design related). Some appropriate design work that you believe will benefit the cause is done and together with an explanation, it is send to them. Completely unsolicited way of helping a good cause just because you felt like it.

rad- short for radical, which meant cool, a term that is still used today by a populace of individuals that never really grew out of the 80's scene and still believe that other terms are cool such as "hey dude" or worse "dudette" and even "totally." Beware of these individuals for they are probably still sporting a Madonna or Micheal Jackson glove on one hand with the fingers cut out of it, watching movies such as Pretty in Pink or The Breakfast Club, and own a very colorful collection of wrist or ankle warmers.