Trainer experience

During the game, Trainers gain Experience Points (XP or EXP for short) which results in their Levels. Trainer's experience is basically measurement of their progress of gameplay.

Experience Points
A Trainer's advancement is measured in Experience Points. As a Trainers gain more XP, they will advance to higher level. XP can be gained through capturing Pokémon, hatching Eggs, interacting with PokéStops, and battling Gyms.

List of Experience Points sources
There are several ways of obtaining Experience Points. If Trainer is under the effect of a Lucky Egg when performing any of actions that rewards them with Experience Points, the XP gained will be doubled.

Trainer levels
Trainer's Experience Points are measured by Levels. The higher certain Experience Points values Trainers have, the higher level they advance to.

A Trainer can level up to access newer items and more powerful Pokémon. The current maximum level a player can get is 40. Each time players level up, rewards are given to them based on what level has been achieved.

List of levels
Each level requires a certain amount of XP and achieving it rewards Trainer with certain items.

Trivia

 * Unlike in the main series games, Trainer level is more important than a Pokémon's level in Pokémon GO.
 * In fact, Pokémon levels are replaced with CP values that partially informs player about their Pokémon's level.