excuse me if this is a stupid question.
Suppose I check out FolderA from svn, and there're plenty sub-folders and files beneath FolderA, right click FolderA, TortoiseSvn--show log, I can see several revisions listed there, what does these revisions mean? if I change a file or a sub-folder under FolderA and submit it, will this result in a new revision of FolderA? if I click one of these revisions and select "revert to this revision", what will happen?
thanks in advance.
It depends.
First of all you need to do some research about Version Control Systems (like SVN or Git) to understand how they work.
If you check out FolderA and then commit your changes to the same branch from which you checked out then yes you will override the previous changes so in a way you will create a new revision.
If you have low experience on version control systems like svn you can think (for now) the revision as an ID for a certain state of the project.
NOTE: It would be a waste of time to explain you in detail what revision is. If you still want to learn about revisions look at this link here
or search google =)