'System Values' are special global variables that provide information about the state of the current player and the world. e.g. 'gPlayerAge' tells you the age of the current player. You would get this value as shown
$myvar = $gPlayerAge
Here is the full current list of System Values:
gPlayerID
gPlayerTeam
gPlayerLevel
gPlayerKudos
gPlayerFamily
gPlayerCash
gPlayerVehicle
gPlayerFuel
gPlayerAge
gPlayerFollowing
gPlayerNumFollowers
gPlayerKills
gPlayerDeaths
gPlayerRobocrowKills
gPlayerRobocrowDeaths
gPlayerItemCount
gPlayerHealth
gPlayerHunger
gPlayerThirst
gPlayerMaxHealth
gPlayerX
gPlayerY
gNearestBuilding
gServerTime
gNumPlayersOnline
gBuildingAccessNum
gBuildingScriptParam
gKillerID
gKilledID
gTargetID