Restarting an iPhone, or in this case a "Soft Reset", as it is also called, restarts the phone automatically and refreshes settings and memory on the phone. This is not something you should do often, but in some cases it can help solve some problems.
Unlike a "Hard reset" you do not lose anything by doing it, all your Apps, Settings, Pictures etc. remain where they were before and as they were before.
It is different how it is done on the different iPhone models, so we have chosen to link directly to the Apple Support's article, that shows you how to do it.