When a volume of air is heated, it expands, if the volume is not contained in a rigid container. If the air is in a rigid container, the internal pressure increases against the walls of the container.
Heating up a volume of air causes its molecules to vibrate faster and occupy more physical space if there is no restraining containment. For each 6 degrees Fahrenheit temperature increase, the volume will increase by about 1 percent. The density of the air will decrease as the volume increases. This is why hot air rises, and cold air descends in a convection cell. The same idea is used in hot air balloons. The hot air within the balloons has a lower density than atmospheric air, and it pushes the balloon upwards.