Here are all the meanings of “high” as a noun:

noun

1.     A lofty place or region.

2.     A lofty level or degree: Summer temperatures reached an all-time high.

3.     The high gear configuration of a transmission.

4.     A center of high atmospheric pressure; an anticyclone.

5.     Slang. An intoxicated or euphoric condition induced by or as if by a drug.

Excerpted from The American Heritage® Dictionary of the English Language, Third Edition  © 1996 by Houghton Mifflin Company.

Depending on the context of your given sentence, either definition 1 or 5 could fit.

In the same sense of the meaning, you might hear about a "runner's high" which apparently some of them get after running for a long distance. (Me, the most running I do would be trying to catch the ice cream truck, but some people like to do it for fun.) 

Also, you can refer to peoples' moods (especially those who have bi-polor disorder) as "highs" and "lows," feeling very happy and energetic on a high and very sad and lathargic on a low.

But in your context, it COULD be drugs (you've seen the whole thing) or it could be referring to someone who just went parachute jumping or white water rafting or something very exciting that produced this euphoric condition.