how to loop only a particular section
You can't. Either shorten the tab you're working on, or make a copy of the tab and delete the parts you don't want to loop.
Simply save the tab as a different name, like "[your tab name here]-copy". It will save another tab that has the same things as the original, but a different name.
but how do you save the tab with a different name. and by the way i do not understand what is a meant by a tab. Is it the name of the song ? I can see that i can create many lines for each song, but are these lines called tabs ?
Click on the "save" button, and change the name.
In Hookpad, a "tab" is simply a file that has a melody/chords. You can have multiple tabs per song, or a song per tab, or something else. All that matters is that a single tab is a single file.