High Sec:
High security systems is the safest security level as it is secured by concord which prevents PvP [Player vs Player] combat. In high sec systems, concord prevents you from locking other players' ships. The only way you can lose your ship is to NPCs in PvE [Player vs Enemy] content but you are safe from other players.
It is the ideal place for new players to play the game as they learn how to play. It is strictly advised to new players to not go into low security because of the dangers which lie within them
High sec ranges from 1.0 to 0.5 security levels and is indicated with cyan and green colours
Low Sec:
Low security systems are less secure than high security systems. Concord only exists to a certain point in these systems. You can only find concord forces at stargates or stations which means that you can engage in PvP [Player vs Player] combat in other parts of the system like Cosmic anomalies, Mining belts, etc.
Low sec ranges from 0.4 to 0.1 security levels and is indicated with yellow and orange colours
Null Sec:
Null security systems are lawless spaces where concord is not present which means you can get attacked by players anywhere in the system including stargates and outside stations. The only safe place in null sec is being docked in a NPC station
It is strong not recommended for new players to be in null security systems as you will become an easy target for more advanced players
Null sec ranges from 0.0 to -1.0 security levels and is indicated with red