For an aircraft to maintain level flight, a force directly opposite and equal in strength to weight is needed. This force is created by the airflow over the wings and is called lift.
In this diagram, the weight is shown with a vector arrow pointing down to towards the centre of the Earth and the lift force is a vector arrow pointing perpendicular to the relative airflow.
As these two are directly opposite each other and equal in strength (shown by the arrows being the same length), the aircraft will maintain level flight.
If lift were less than weight, the aircraft would descend and if lift became greater than weight, the aircraft would climb. So we can see that controlling lift has a large effect on our control of the aircraft’s flight path.