The black and white flag is displayed to indicate that a driver has been unsportsmanlike. Using the situation at the weekend, it was determined that Leclerc was unsportsmanlike in the way that he forced Lewis Hamilton off the track.
The black and white flag is most commonly used when a driver purposely forces another car off the track, or if a driver gets out of the car and starts an altercation with another driver.
The flag can also be used as a warning for when a driver exceeds track limits and has all of their wheels over the white line.
The flag is essentially a warning. Like a yellow card in Football. There is no penalty for getting a black and white flag but if you don't change your actions and continue to drive in an unsportsmanlike manner then you will be black-flagged and disqualified from the race.