If it is not then you will not be able to boot into your computer! If your BIOS mode shows as legacy make sure your computer is capable of UEFI first. For me, it showed as legacy, and later it was changed to UEFI after I implemented MBR2GPT. Go to the search bar, and type in "MSINFO32", and see what your BIOS mode is. Make sure you write this down somewhere or remember it, because you will have to use it later. See which drive is your boot drive, the drive that has your operating system on it or Windows 10 on it. Go to the search bar, and type in "Computer Management" then select "storage", and "disk management". Thankfully, upgrading from MBR to GPT is not difficult to do on a computer with a UEFI BIOS.
In addition a GPT partition is more secure and faster as well. The maximum limit for a storage device with a legacy BIOS is 2 TB or 2000 Gigabytes, on a UEFI BIOS you can use storage devices over 2 TB. It has limitations on how much space you can use on any given storage device. Legacy BIOS was created in 1982, and it is very old. If this is the case then this video is useless to you, and if you attempt to use this then you will not be able to boot into Windows 10. If you have an older computer made before 2013 then your computer will not have a UEFI BIOS. In order to use UEFI, you must enable it in your BIOS. Most motherboards which were produced from 2013 and later have UEFI BIOS, but they can boot from both legacy and UEFI.
Having a UEFI BIOS enables you to use programs, like bitlocker which is a application that encrypts your storage device. UEFI has more security features than the older legacy BIOS.
In order to get a UEFI BIOS, you need to use the newer partition called GPT(GUID Partition Table). When you install Windows 10 on any computer by default, the partition you get is MBR(Master Boot Record) which is tied to a legacy BIOS.
This video shows you how to enable UFEI BIOS on Windows 10.