How the poineer astimate its position in the world?

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Hello,

I have been using the Pioneer3AT and I run RosAria in order to get its position but I never understand how ti finds the position and what is the world reference frame to it. When I looked for the explenation I found the following:

(This pose estimation is calculated by the robot firmware as ARIA based on wheel odometry, and if preset, the optional internal gyroscope.)

I also found this as comments in the code of Aria

• This position is updated by data reported by the robot as it
• moves, and may also be changed by other program components,
• such as a localization process (see moveTo()).

• This position is also referred to as the robot's
• "odometry" or "odometric" pose, since the robot
• uses its odometry data to determine this pose; but it
• may also incorporate additional data sources such as
• an onboard gyro. The term "odometric pose" also
• distinguishes this position by the fact that its
• coordinate system may be arbitrary, and seperate
• from any external coordinate system.

I didn't understand it ?? is the (0,0) position is the point where we place the robot and we first open it ?? can any one explain please ?