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Both turtle1 and turtle2 broadcast their current pose as a tf transform (a coordinate frame). The pose of turtle1 is driven by keyboard commands, while turtle2 tries to follow turtle1. It does so by setting up a tf listener and computing the transform of turtle2 with respect to turtle1, which serves as error signal for a simple proportional control on twist coordinates.

More general information on what the tf package does, and how it works can be found in the tf ROS wiki page.