Explain in detail what each node does to which topic it sends, I don't understand anything
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@miura: +100 for posting a response.
Tbh, I would not expand on your answer, unless the OP provides more context and shows some more effort.
Simply plonking a screenshot of rqt_graph in a post and then demanding everything is explained is not what we consider a proper question here on ROS Answers.
To further clarify on the given example:
move_base is publishing a "twist" message on the /cmd_veltopic.
This is a Twist message: geometry_msgs/msg/Twist
How do I know this will be a Twist message? Simply put: that is just what the community has settled-upon. When you want to make a robot move you do it by sending a geometry_msgs/msg/Twist message on the /cmd_vel topic.
As you can see in the flowchart there is also a turtlebot3_teleop_keyboard node publishing Twist messages on the /cmd_vel topic. This isn't a conflict; this is a feature of ROS. Multiple nodes can publish the same messages on a topic, and any number of nodes can subscribe to that topic.
That's not to say that there won't be a conflict. /cmd_vel is telling the robot "You must travel at V m/s with a rotation of W r/s" so ...
Another important one to point out is that robot_state_publisher on the right side. As you can see from the flowchart it is publishing a /tf_static topic, but what might not be immediately obvious is that robot_state_publisher is only publishing that topic.
I say this because there are three arrows originating from that node. If you look closely they are all given the same /tf_static label. What this tells us is that there are three other nodes subscribing to that topic:
amcl on the far left side, turtlebot3_slam_gmapping also on the left, and move_base in the center-bottom.
/tf_static is a topic, but it is referring to a Static Transform. I'm running out of space here, but here are some links explaining Transforms in ROS2:
