When you fly you will sometimes notice that the plane wobbles a little. For example, if the plane flies through an air pocket it can drop a few metres very briefly. It feels like you are sat on a roller-coaster. Of course, there are not really air pockets in the air like holes in a piece of Swiss cheese because we are surrounded by air everywhere. The term "air pocket" refers to a really fascinating, natural phenomenon.
You might have noticed it before in summer: whilst it is pleasantly cool on the ground floor of a building, it is hot and sticky on the top floor. There is a simple reason for this: cold air is heavier than warm air. So it flows downwards. The warm, lighter air then rises up.
That means that air is constantly moving, even really high up where planes fly. Vertical movements of air are also referred to as upwinds and downwinds. So, when warm air flows upwards (upwind), the cold air has to move downwards at the same time (downwind). If a plane flies through an area where cold and warm air meet, the cold air not only suddenly streams downwards; the plane is also pushed downwards.
As a passenger, it then feels like the plane is falling into a hole. The pilot only has to accelerate a little and the plane returns to the height it was at before.