To understand the concept of Origin field from map_server

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Hello. I'm trying to run some tutorial which have a ROS Navigation Stack sample. On doing, I couldn't understand the concept of ORIGIN field from yaml which the map_server create after mapping.

Here is a description by ROS wiki.

origin : The 2-D pose of the lower-left pixel in the map, as (x, y, yaw), with yaw as counterclockwise rotation (yaw=0 means no rotation). Many parts of the system currently ignore yaw.

What this, "the lower-left pixel in the map", mean??