Why does my computer shut down after five minutes of being idle without sleep or hibernation turned on

When it is on, and you are not doing anything on the computer, a pop up will come up and it will say shut down, restart, or cancel. We do not know what this means or how to fix it. If we do not do anything on the pop up, it will automatically shut down. Hiberation and sleep are both turned off. It is an original MacBook running OS X.