Stamina is a new feature introduced in Version 1.12.0 of Pokémon Masters. With this, to complete some stages, you need to use Stamina. The stamina you use is determined by the type of stage and the difficulty of the stage ranging from 10 Stamina Points up to 25 Stamina Points.

The amount of Stamina you have is based on your current Player Rank, which increases as you beat more levels.

Stamina Image

Replenishing Stamina

There are four ways to replenish Stamina: Wait - Stamina refreshes as time at a rate of 10 Stamina Points every hour
Purchase - You can spend 40 Gems to fully refill your Stamina gauge
Stamina Refill Items - Sometimes in various rewards for Missions or as part of the Daily Rewards you could get Stamina boost icons
Rank Up - When you rank up your Player Rank, your Stamina gauge gets refilled.

Stamina Image