What's "rock out" and "watch on"?
I was using Khan Academy this morning, I saw the following graph
http://postimage.org/image/wt4gytbab/
What's "rock out" and "watch on"?
Could you please be kind enough to tell me phrases like this, how do I look them up in dictionary?

Question

Answer

These are very hard to look up in a dictionary.  They are slang terms and not very good ones at that.

Rock out - it is a slang that started in the 1970's or 1980's and usually refers to enjoying rock and roll music or partying.  It is also said as "Rock on" which means to keep on rocking, or listening, or partying.  Rock out more than likely here means to listen.

Watch on - This is not really a frequently used slang but it is easy enough to understand as 'keep on viewing' or 'view more'.  Since this is connected with a video on the site you mentioned, I would say this is exactly what it means.

I hope this helps.
Mat

Answer

On Khan Academy, they are trying to be casual and cool with their vocabulary:
- "Rock Out" in this context, is literally "Get Started"
- "Watch On" in this context, is "Continue Watching"

Rock Out especially is more colloquial.