Stamina is the Harvest Moon Games' measure of energy. By using tools, the player loses stamina. Once all of this has been depleted, you will no longer be able to work. Working in the rain makes you lose stamina faster. In some games, there may be a Hot Spring to replenish stamina in, or in some cases, a bathroom.
By eating power berries, you increase your stamina and therefore your ability to work for longer periods of time.
You can tell your stamina in Harvest Moon DS by wearing the Truth Bangle accessory in the blue square. It will say how much Stamina and Fatigue you have. There is 100 Stamina Points and 100 Fatigue Points. If you get to 100/100 Fatigue Points, you will faint.
In Harvest Moon: A Wonderful Life, if your player becomes tired or queasy, you can eat something to make it better, the easiest option is a shroom lying around such as a mugwort or bracken, it will act as if your player doesn't like it but it does the job fine.