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OEM refers to Original Equipment Manufacturer.

Go to Settings>About Phone & tap Build Number 7 times (assuming you are running Android 5.0 or later), a hidden menu called Developer Options will appear. In this menu, you will find (unless the manufacturer blocks it) an option called OEM UNLOCKING. Enabling this feature makes absolutely no change at all to your device, except for giving you the option to manually unlock your bootloader. This does not unlock your bootloader, but allows you to manually unlock it via a tethered command prompt if you so choose.
If you enabled OEM UNLOCKING and then manually unlocked your bootloader using a tethered fastboot mode command line. So now that your bootloader is unlocked, what changes? Well as far as your Android OS goes, nothing changes. Unlocking the bootloader removes a layer of protection which allows your /boot, /recovery, /system, /modem & other partitions to be flashed. This is not rooting and not the same as rooting.
Therefore, an unlocked bootloader does not alter your /system partition or your Android OS directly, as others have erroneously stated. But, you can now flash your device with a custom recovery which will allow you to install a custom ROM, a custom kernel, or flash a root script to your stock Android OS, among many other things. Bootloader unlocking & rooting are two separate & distinct procedures and the two are not dependent on one another. In other words you can fully root a device with a locked bootloader, just as you can have an unlocked bootloader and have a non-rooted device. Of course, to have full customization options, rooting and bootloader unlocking are preferred.

As far as your warranty goes, you can void it without unlocking the bootloader. Again, you can achieve full root without OEM UNLOCKING, which can void your warranty. Just as a bootloader can be unlocked, it can be relocked. Same with rooting. It can be removed as well. Most devices which have been modified and/or OEM unlocked can be restored to stock factory condition, with no trace that any modifications were ever made.