@oristarA,
A layover is a rest stop for air travel passengers who have to take more then one plane to get to a destination.
Example:
Joe needs to get to Fairbanks, Alaska from Boston.
Takes a plane from Boston to Seattle. Then another plane from Seattle to Fairbanks, Alaska.
The layover would be the waiting time for Jim at the Seattle airport while he waits for the boarding of the plane to take him to Fairbanks.
I guess if the layover time is long enough (planned or otherwise), Joe could find something to do (layover activities) near the airport or around the city of Seattle before he has to return to the airport for his flight to Fairbanks, Alaska.