Aerodynamics means studying how air (or gas) travels around something moving through it.
The study of gases that are not in motion is called aerostatics.
Aerodynamics comes from Aero (Air), and Dynamic (Moving).
In planes, lift is caused by airflow. Airplane wings are designed in such a way that, the air molecules prefer moving under the wing rather than above it, so as a result there is more air flowing underneath the wing compared to above it. This motion of the air grants upward lift to the aircraft. See Bernoulli's principle.
Thrust and drag decide how an object moves forward. Thrust is the force pushing forward, and drag is the force pushing backward. Thrust is usually made by either propellers or jets. Drag is the force pushing backward. Most drag is caused by air friction.
Images for kids
Forces of flight on an airfoil
Aerodynamics Facts for Kids. Kiddle Encyclopedia.