The black plastic is to keep unwanted plants (i.e. weeds) from growing. In the early season it also captures sunlight and heats the ground, as well as helping the soil retain moisture. The seeds you plant in the slits will get plenty of light as soon as they break through the ground.
If you are going to leave the plastic throughout the season, make sure it is perforated to allow rainfall to reach the soil. The basic concept is that you are creating a ground level green house. Good luck with your garden this year!