Generally security can be divided into two classifications, either Core or Border.
Core sectors factional rules and regulations are strictly enforced, whereas Border sectors tend to be more lax about enforcing faction laws. The capital sector of a race is always Core, and most sectors around will also be. Major colonies and areas nearby will be marked as Core, whereas further-out sectors will be Border.
The security can be viewed from the Universe Map.
When a player's faction ranking is low (approximately -1), they will be allowed in that race's border worlds, but not core worlds. Otherwise, the only major gameplay differences tend to be the frequency of enemy raiders and the number of allied patrols.
The difference in X3: Reunion was quite noticeable as the player would not be allowed into Paranid Core Sectors for a good portion of the game. In X3: Albion Prelude, the difference is somewhat lessened by the war going on throughout the game.