Newton's First Law of motion states, "Every object continues either at rest or in constant motion in a straight line, unless it is forced to change that state by forces acting on it." Inertia derives from the First Law of Motion and it is, " The tendency of an object to remain at rest, or to keep moving once it is set in motion."
As you can see in these demonstrations, inertia causes an object to remain at rest or in the same position as long as the object is not directly affected by an external force. An external force must push the object with more force than gravity is imposing on it. The less mass an object has, the less inertia it has. Once the mass increases, so does its inertia.
We can easily see Inertia in everyday life. Let's say the use of a car. You are driving at a constant speed of 40 MPH. Then, suddenly the car in front of you makes a sudden break. You follow along and this causes your body to move forwards once the car goes to a complete stop. This is caused by the inertia your body has. Your body it is being pushed back as the car is going 40 MPH, but when the car makes that sudden stop your body continues moving forward at a speed of 40 MPH and if it weren't for seatbelts we would possibly fly out the window.