What are the roles of <param> tags within the <node> tags in launch file?

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Thank you so much for your answer, so the <param> tags within <node> tags are like the formal parameters of a function in programming language, am I correct?

You can say that but it's more like if you values in differents nodes you need to adapt for your application, instead of changing the source code of each node you will change those parameters in your launch file. It allows you to see all your parameters used for each node in the same file.

But be aware that you have to define this in the source code, with the same example if there wasn't this line in the code :

n_tilde.param("publish_frequency", publish_freq, 50.0);

You wouldn't be able to use <param name="publish_frequency" type="double" value="30.0" />

Additionnaly there is an <arg> tag within the launch tag allowing you to pass arguments in command when executing a roslaunch. For example you might have seen this :

roslaunch urdf_tutorial display.launch model:='$(find urdf_tutorial)/urdf/01-myfirst.urdf'

The model part is an argument.

Thank you for your guidance.

<node name="urdf_spawner" pkg="gazebo_ros" type="spawn_model" respawn="false" output="screen" args="-urdf -model cambot -param robot_description"/>

I also like to know what this args attribute stands for.

You should always check the wiki (the tag reference part) it's all detailled for each tag. The args is the same thing as my previous comment but instead of typing it in the terminal you define the arguments directly in the launch file.

I've read the page you provided, but it does not seem to solve my question. It just gives an example of <node name="add_two_ints_client" pkg="beginner_tutorials" type="add_two_ints_client" args="$(arg a) $(arg b)" />, but without explanation.

Then I try to read this page, it simply gives a statement of args attribute() without further explanation. Here is the explanation of this page about args:

args="arg1 arg2 arg3"(optional)

Pass arguments to node.