Lift is a component that keeps a plane in the air. This is the upward force that fights the gravity/weight of the plane. It works basically as the wind flows over and under the wing. As the air over the wing goes faster and under goes slower, it will combine at the end of the wing.
The atmosphere is the layer around the earth, which is around 100 to 200km from the ground up. Worldwide, we use the International Standard Atmosphere which is a mean set of conditions which will be somewhat different depending on the weather conditions, location etc. This is a mean of the conditions at 45 degrees north latitude.
In aviation, we are mostly in environments of low air pressure. As we go further up in the air, the pressure goes down. This will have some effects on humans, where we could have hypoxia and barotrauma.
Navigation and Flight Planning is a very important part of preparing your flights and knowing where to go in the air. When a flight is not properly prepared you can get several different kind of issues. These are the most pilot-caused problems in the air: