All character features are determined by the character's Level. The Character's Level is determined by the starting level of the campaign and how far the characters have progressed. Depending on the starting level of your character, you may have access to different things. The lowest level you can start at is level 1, and the highest overall level you can reach is level 20. There are two main ways that characters can obtain levels, or "Level Up" in Dungeons and Dragons.
Experience is the default way to level up. Players obtain Experience, or XP, by defeating enemies in combat, or as a reward by the Dungeon Master. The character must reach a certain amount of XP to level up, which is determined by the overall level of the character.Â
Every time the character obtains a level up, their XP resets to 0, and a new maximum XP is determined by the new overall character level. This makes it to where players know exactly how far they are from leveling up, but could lead to "XP farming", where players will seek out combat in order to quickly gain XP.
Milestone is the custom way to level up. Instead of combat XP determining a character's levels, the Dungeon Master decides when the players advance to the next level. This can be based on campaign progression, frequency, or any other factors that the Dungeon Master decides.
This removes the calculation factor from leveling, and prevents the players from attempting to "XP farm". However, this also leaves exactly when players will level up entirely to the Dungeon Masters, and can also leave players uncertain on their progress towards their goals.
Dungeon Masters may come up with their own methods of determining how players can level up their characters, such as point systems or through progress points. These are simply the two officially recognized methods by Wizards of the Coast.
When it comes to multiclassing, taking on another class reduces the maximum level of the other class. When attempting to multiclass a character, players should be careful of what levels they want their character's classes to end as in order to make sure they can make the most of their builds. This is why many Dungeon Masters will have a "Max Level" in mind for their campaigns. Level is also important for accessing certain abilities. Most subclasses are only available after reaching level 3 in that class, while certain race abilities such as racial spells can be locked until you reach an overall level. Certain magic items can also be level locked, as they may require a minimum level in a class, subclass, or overall character level.