What is a thermal in hang gliding?

A thermal is a mass of coordinated ( i.e. a distinct bubble ) air which rises due to it being warmer than the surrounding air. On most days these bubbles continue upwards ultimately forming Cumulus clouds ( Cumulus or the larger Cumulonimbus clouds are themselves thermals made visible )

Think of the atmosphere as gently boiling ( bubbling) as the day warms up ( the sun heating the ground ) and the lid is the inversions ( where the air gets warmer).