In the exercise demonstration and tutorial videos, you’ll see that Marc is using a double overhand grip for deadlifts, which is how he teaches proper form to a beginner.
However, as you lift more weight, we recommend that you use a mixed grip - which means one hand is underhand while the other is overhand on the bar. A mixed grip can enable you to lift upwards of 50 pounds more than a double overhand grip. This is because a mixed grip helps prevent the bar from rolling out of your hands. Which hand you choose to go underhand is up to you. Consider alternating your grip between sets to prevent muscular imbalances.