Your multirotor will only fly properly when the correct propellers are installed on their corresponding motors. Get this wrong, and your quad will flip right over on take-off—a maneuver sometimes called “pancaking.”
The angled blade pushes downward on the air if it is rotating in the proper direction. This downward push generates lift:
Check the top chassis plate for the direction your propellers should spin when installed correctly. Remember: do not have propellers installed while doing any pre-flight calibrations or safety checks; spin direction can be assessed by watching the motors (without propellers!) during calibration.