Imagine you are driving a car with the window open and you stick your hand out in the airflow, with your hand such that you are knifing through the air making the least resistance to the airflow.
If you now tilt you hand to create an angle either upwards or downwards then the airflow will push your hand up or down.
This is lift - when you hand is angled upwards and downforce when it is angled downwards.
The lift / downforce slider adjusts how strongly this aerodynamic effect acts on the quad (in both directions equally).