Unlock bootloader Galaxy S3 SGH-T999

[loki431]

Hello folks, I'm a beginner about rooting devices,
well, that's what is happening
I followed this tutorial
How to Root Galaxy S3 (T-Mobile) SGH-T999 on Android 4.1.2 Jelly Bean Firmware - Honai of Android
and rooted my device ( Galaxy S3 SGH-T999 )
then I wanted to install a ROM (first time doing this), after a long time choosing, I went for this one
★[ROM][6/14] Hyperdrive RLS14 ║ ★ Unmatched Customization and Tweaks ★ ║ - xda-developers
and it says I need an Unlocked Bootloader
but what is it?
How can I get it ?
I need some help please ! :c

 

[steeletown]

Your bootloader should be unlocked. Just follow the directions on flashing the hyperdrive rom, but make sure you backup your stock Image before you flash.