Hello. The term "hard reset" means that the software on the phone is reset to factory status. It will be like the day you bought it. All apps, data, settings are deleted. If you have signed into your phone with a Microsoft account, much of this should have been backed up for you. Your text messages, installed apps, pictures, and theme can all be backed up. Go to settings > backup to make sure that all of these things are backed up, and that the backup is current (tap apps+settings in backup to see how current your backup is).
The backup will not save many of your in-app settings. Game progress will be lost, for instance. The actual MP3s stored on your phone will be deleted, and you'll need to add them again, but your playlists will backed up to the Xbox music service so you can synchronize playlists amongst compatible devices.
If all of that is ok with you, and you consider yourself fully aware of the consequences, go to settings > about. At the bottom of that screen is the button to reset your phone. Tap that, and it will give you a warning box to make sure that you're sure, and the process will begin.The phone will reset and come up with the gears screen, and maybe 5-15 minutes later, you're phone will restart again and you're done. You'll need to sign in with your Microsoft account, and you'll be prompted to restore from backup or start new. Choose the restore option, and you're settings and apps will push down to the phone. It may take a while, but your phone should end up in nearly the same condition as before, hopefully without the glitches.
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