If you can run for a really long time, or carry a heavy box a really long way, you have stamina. Stamina is staying power or enduring strength.
Stamina is not always related to physical strength and endurance. Solving a difficult puzzle or a complex problem requires your brain to work long and hard, something called mental stamina. If you have had a really tough year, but are doing generally fine, someone might say you have emotional stamina. It can also mean perseverance. Do you have the stamina to learn this word?