"Thermal" is the name given to a bubble or column of warm rising air common on most sunny days. It is the sun itself that heats the ground and the air that is near the ground. This warmth makes the air a bit lighter than the air above it causing it to rise in a manner similar to bubbles rising in a glass of soda.
An important skill in soaring a hang glider or paraglider is to plan one's flight path so as to maximize the time spent in the thermal and to stay as close as possible to the strongest part of the thermal (usually the center). Often there are many pilots in the air at the same time. One should understand and obey the right-of-way rules.