Security Ratings

Overview
The Security Rating of a system determines the "Lawfulness" of the system. This mostly has to do with how large the CONCORD presence is within the system which, in actual game mechanic terms means how fast they respond to bad things happening.

In general, most named systems such as Jita, Amarr, or Keberz will have a positive security rating while names such as HED-GP or K-6K16 will have a security rating of 0 or lower (more on this in a second).

Actual Game Mechanics
So, there's a few things that will actually impact your gameplay. Even if you get the hell out of Empire when you join Kite Co. sooner or later this is going to matter to you.

Space Police

 * 1.0 - 0.5: Faction navies (Caldari, Gallente, Minmatar, Amaar), CONCORD, and sentry guns on stations and gates will respond to characters who are criminally flagged and who have a low enough standing with their respective faction.
 * 0.4 - 0.1: Sentry guns on stations and gates will respond to characters who are criminally flagged or have low enough standings with their faction.
 * 0.0: Sentry guns at NPC-owned stations will respond to criminally flagged characters.

CONCORD
CONCORD (Consolidated Cooperation and Relations Command) is the highsec Internet Spaceship Police which is the primary game mechanic to discourage Suicide Ganking. Surviving a Concord attack is not possible and thus considered an exploit. CONCORD response times are as follows:

They will show up promptly to the first attack in a system, but their response to subsequent attacks (after spawned in system) will be delayed by approximately 6 seconds. CONCORD despawns at downtime.

Pirate NPCs
I doubt you'll be asteroid belt ratting in Empire space. Long story short, the lower the security status (this includes truesec) the larger and more "powerful" (read: lucrative) the pirate NPC spawns and the associated cosmic anomolies and signatures become. A -0.83 system will produce better spawns than a -0.21 system.

Anchoring

 * 1.0 - 0.9: No anchoring fun allowed. Period.
 * 0.8: You can anchor containers in space.
 * 0.7 - 0.4: POSes can be anchored at moons with the right amount of standing:
 * Basically, you need +1.00 standing per 0.1 security rating of the system. So, a +7.0 standing with the Sovereignty holder in a 0.7 system all the way down to +4.0 for a 0.4 system.


 * 0.3 - 0.1: POSes and containers may be freely anchored and moon mining equipment can be set up.
 * 0.0: Normally this would be the same as the other 0.3 systems on down but there is one minor thing you should be aware of and only applies to player-owned systems. If you are not a part of the alliance that holds sovereignty in a system they will receive a notice that you are anchoring a POS in their system.

Restricted Systems
Ah finally we come to your security status. Remember, it goes up when you shoot pirate NPCs (rats) and down when you shoot other players (not in your corporation or fleet) in low- or high-security space. The thresholds for provoking a Faction Police response are as follows:
 * 1.0: -2.0 and lower.
 * 0.9: -2.5 and lower.
 * 0.8: -3.0 and lower.
 * 0.7: -3.5 and lower.
 * 0.6: -4.0 and lower.
 * 0.5: -4.5 and lower.
 * 0.4 - 0.0: No restrictions.

So, if you have a -3.2 you can go into systems that have up to a 0.7 security status. The Faction Police will blow up your ship if you go into a 0.8 or above though.

While we're on the matter of NPCs blowing up your ships, we might as well take a second to discuss faction standings and how they affect the police NPCs as well. If you have a -5.0 standing or lower with the faction and you try to enter a system that has faction police then they will try to web and then scram your ship. You can possibly get away in a fairly agile ship.

True Security
A while back all of 0.0 space was just displayed as having a 0.0 rating. However there was a "hidden" true security status that went all the way down to -1.00. The lower it went, the better the asteroid belt rats spawned. More recently, however, CCP actually made all of the "hidden" truesec displayed on the system information in game. Not that it wasn't hard to find anyways, though. Dotlan has been able to pull up the truesec of a 0.0 system for as long as I can remember.

Anyways, there is one final caveat. Due to developer events or some other dumb reason, each of the major pirate NPC factions (Angel Cartel, Sansha's Nation, Guristas, Serpentis, Blood Raiders) have a "home" region. The Venal (Guristas), Fountain (Serpentis), and Delve (Blood Raiders) regions have what is called "broken" truesec. Basically, the game treats all systems within those regions as having a -1.0 rating for the purpose of spawning asteroid belt rats, faction spawns, and officers. Sansha and Angel ratters however, do not have such a luxury.