If something is described as having a high concept, it means that it's got a premise that can be easily pitched in a succinct sentence. Moreover, it's got a premise that's immediately appealing, indicating that it's something an audience would like to see, regardless of what the rest of the story is about, or who might star in the film (or TV show).
High concept examples:
After his son makes a birthday wish, his father, who's a hotshot lawyer, has to tell the truth for 24 hours, just at a crucial time for him at work.
A LA cop races to save a bus in the middle of rush hour as it's set to explode if it travels less than 50mph.
An American in Iraq awakes to find himself buried alive in a coffin, with only a mobile phone to help him escape before his oxygen runs out.
Snakes on a plane.
Concept (blog 2005)
High Concept (Absolute Write)
High Concept Defined (Writers Store)