Well, my first question is, what are Hash Collisions? In OBMM, I've activated Qarl's TP3, and went from green to blue icon, did the whole 'archive invalidation' thing, and received 'Hash Collisons: 1'. Is this of any importance?

For archive invalidation, just select BSA redirection and be done with it. Blue means that the BSA is activated. Green means that the BSA is deactivated and has no conflicts. The Hash Collision 1 is for another type of archive invalidation. It is fine, but do not use that. Turn on BSA Redirection instead, and forget about archive invalidation from here on out.

Ah I see. And, what about Activating and deactivating? I have Qarl's TP... If I deactivate it, will that mean it'll revert back to original textures all together?

Yes. Why would you do that though? Are you confused about the meaning of "activate" and "deactivate"?

Edit: I highly suggest using QTP3 Redimized instead of QTP3, unless you have a beast of a system.

http://sites.google.com/site/oblivionpoinfo/prep/obmm <-- See the intro to the interface page too.

Well, I installed Qarl's TP3, and also the Detailed Terrain MOD, but didn't see any difference really and didn't want any conflictions to occur (mod-noob). But when I installed Detailed Terrain mod, it overwrited some of the Qarl's files, asking me to replace, so I said yes to all. But if I deactivate, would that no restore the original Qarl files?

QTP3 makes a huge difference. If you did not notice a difference, you do not have archive invalidation setup properly. Detailed Terrain should indeed be installed on top of QTP3. OBMM would not restore QTP3's files after deactivating the DT mod because of the way it works. Wrye Bash's installer, BAIN, would, however. For now, deactivate both of those BSAs, and restart OBMM (to get fresh memory allocated.) Then restart OBMM, and, after making sure that BSA redirection is set in Utilities>Archive invalidation, activate 1) QTP3 and 2) DT. Hold down shift when clicking "yes" while installing DT for a "yes to all" effect.