High Season is also known as Peak Season. It is the period of consecutive months during which optimum revenues, room/suite Occupancy and average Room rates are generated.
This is often over the summer months, or school holidays.
See also Low Season.
Other definition:
High season is the time of year when a destination gets the greatest crowds, and thus can increase hotel and rental car rates, etc. As an example, summertime is high season for travel from Europe ( just check the airfares ! )