I found a sector that has backdoor by it kinda by Sd. What does this mean and how can it benefit me?

You say you found a sector that has a backdoor, but you don't know what a backdoor is? I'm confused.

As far as how a backdoor might benefit you, they are very useful for looking into another sector without allowing people in that sector to look at you. So if you expect someone to go through that sector, you might run a density scan photon script from the backdoor.