The very technical explanation:
The NAND partition of the phone is unlocked, and then gives you access to flash even radios and hboots. And also to be able to REALLY mess it up.
Does the htc bootloader unlock gives us s-off? No, that unlock “only” unlocks system and recovery and such. It gives you a way to flash a custom recovery which you then can use to flash a custom rom, but you can’t flash custom hboots and such since you don’t have have access to the NAND partition where hboot and such things are. And this is a GOOD thing for most people.