Memory level - Unsure...I forget if this one sets the gore level or if it's Memory Access that does.
Hide Second Story Tiles - causes buildings and trees and such so they don't block your view
Client CPU Affinity - For multi-CPU/multi-core systems, NWN can run on one CPU or another rather than what what the system wants to do by default.
Enable Floaty Text Feedback - When you see that text that floats upward, that's what they mean.
Single Player Item Level Restrictions - Enforces level restrictions for use/equipping of items in the single-player game
Single Player Enforce Legal Characters - disallows characters that have been edited my non-mechanical means to get stats, skills, feats and such in the single-player game.