Here is how the Character Advancement works in the DND 5E, Once you move Beyond 1st Level in the game.
As your character goes on adventures and overcomes challenges, he or she gains experience, represented by experience points. A character who reaches a specified experience point total advances in capability. This advancement is called gaining a level.
Characters Gaining A New Level:
When your character gains a level, his or her class often grants additional features, as detailed in the class description. Some of these features allow you to increase your ability scores, either increasing two scores by 1 each or increasing one score by 2. You can’t increase an ability score above 20. In addition, every character’s proficiency bonus increases at certain levels.
How Character Advancement Works in DND 5E Beyond 1st level:
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).
When your Constitution modifier increases by 1, your hit point maximum increases by 1 for each level you have attained. For example, if your 7th-level fighter has a Constitution score of 17, when he reaches 8th level, he increases his Constitution score from 17 to 18, thus increasing his Constitution modifier from +3 to +4. His hit point maximum then increases by 8.
The Character Advancement table summarizes the XP you need to advance in levels from level 1 through level 20, and the proficiency bonus for a character of that level. Consult the information in your character’s class description to see what other improvements you gain at each level.
Character Advancement Table
|Experience Points||Level||Proficiency Bonus|
More details about the salient features of each character, Visit Classes or Races to know more.