I know that / stands for folder, but there is one thing I don't get.
Command to illustrate the problem:
mv ~/Documents/newfolder ./anothernamefolder
Why there is / here? What does it do? Why can't I type:
mv ~/Documents/newfolder . anothernamefolder
As per my comment's link (What does the ./ mean (dot slash) in linux?), and as you correctly think, slash (/) is a directory "representation".
In your two examples, the slash represents different things though:
mv ~/Documents/newfolder ./anothernamefolder
move newfolder from Documents located under user home directory (~), to current directory (./) and name it anotherfolder.
In the second one
touch {jan,feb,mar,apr}_{2017..2019}/file{1..100}
you touch files in number of directories.
So for every directory specified in the command: {jan,feb,mar,apr}_{2017..2019} create files file{1..100}.
Why there is "/" here? What does it do? Why I can't type:
As above, ./ is representation of current directory.
If you used
mv ~/Documents/newfolder . anothernamefolder
command, you'd get an error, as you'd be specifying ambiguous redirections:
$ mv ~/Documents/newfolder . anotherfolder
usage: mv [-f | -i | -n] [-v] source target
mv [-f | -i | -n] [-v] source ... directory
You could use dot . to move, but that would simply move folder called newfolder without changing its name.
$ mv ~/Documents/newfolder .
$ ls
newfolder
/ separates directories (folder is a gui-term).
In the first argument to your mv-command /'s separate ~, Documents and newfolder, signaling that each is a subdirectory of the previous.
The same is true for the second argument, saying that you want to move files to anothernamefolder which is a subdirectory of .. As . (in that context) means the current directory, ./ is unneeded.
Just removing the /, as you suggest, changes the meaning of the command, to say that you also (in addition to ~/Documents/newfolder) want to move . to anothernamefolder. That is not possible and will cause mv to give an error.
/is but are confused with the dot (.)