Stamina means how far you can run.
To be fair, stamina doesn't have to be about running. Stamina could also mean how long you can watch tv without stopping or how many hot dogs you can eat without throwing up or how long you can work without falling asleep.
Basically, stamina is how long you can handle something before it gets too hard and you give up. Most of us have some stamina, but not as much as we would like. But now we can buy these Stamina Crew T-Shirts!
Will these shirts actually give you more stamina? That's hard to say. It's true that they won't magically help you to run faster or watch tv for longer or eat more hot dogs.
But, they are comfortable enough that you won't mind wearing them for a long time. So, in terms of physical stamina, you might still have some work to do. But in terms of shirt-wearing-stamina, you'll be all set.