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The paragraph below the breakdown of that line of code states:

Here we'll set some velocities that will cause the "base_link" frame to move in the "odom" frame at a rate of 0.1m/s in the x direction, -0.1m/s in the y direction, and 0.1rad/s in the th direction. This will more or less cause our fake robot to drive in a circle.

Basically the robot will move those amounts with every time step.

nav_msgs/odometry messages take the following form:

# This represents an estimate of a position and velocity in free space.  
# The pose in this message should be specified in the coordinate frame given by header.frame_id.
# The twist in this message should be specified in the coordinate frame given by the child_frame_id
Header header
string child_frame_id
geometry_msgs/PoseWithCovariance pose
geometry_msgs/TwistWithCovariance twist

You can find information concerning geometry_msgs here.

I hope that answers your question. If it doesn't, comment below...

The paragraph below the breakdown of that line of code states:

Here we'll set some velocities that will cause the "base_link" frame to move in the "odom" frame at a rate of 0.1m/s in the x direction, -0.1m/s in the y direction, and 0.1rad/s in the th direction. This will more or less cause our fake robot to drive in a circle.

Basically the robot will move those amounts with every time step.

nav_msgs/odometry nav_msgs/odometry messages take the following form:

# This represents an estimate of a position and velocity in free space.  
# The pose in this message should be specified in the coordinate frame given by header.frame_id.
# The twist in this message should be specified in the coordinate frame given by the child_frame_id
Header header
string child_frame_id
geometry_msgs/PoseWithCovariance pose
geometry_msgs/TwistWithCovariance twist

You can find information concerning geometry_msgs geometry_msgs here.

I hope that answers In a real situation, a robot driver, like p2os or turtlebot, will extract the position and velocity information (or data from the motor encoders) and will produce the nav_msgs/odometry. If you have built your question. If it doesn't, comment below...