Do not turn off your computer with that physical power button. That is only a power-on button. It is very important that you shut down your system properly. Simply turning the power off with the power switch can cause serious file system damage.
If you momentarily press the power button, it is like selecting "Shut Down". If you press and hold the power button, it is a "Hard" Shut down and will turn off the computer without closing files/apps like a regular shut down does. This can corrupt files and potentially damage your computer.