In D&D5, players have the choice of rolling their Hit Dice or using a fixed average value instead:
Each time you gain a level, you gain 1 additional Hit Die. Roll that Hit Die, add your Constitution modifier to the roll, and add the total to your hit point maximum. Alternatively, you can use the fixed value shown in your class entry, which is the average result of the die roll (rounded up).
If I roll poorly for the Hit Die, can I ignore the roll and use the fixed value instead?