X-plore allows to work with multiple files or folders at same time. Some (but not all) operations work on selection of marked files.
Marking selection is done by check marks located at right side of listed files.
In X-plore's tree view, where you can see complex folder hierarchy at same time, marking of multiple files works always only within one folder. So you can mark multiple files that are all placed in same parent folder.
As you'd try to mark file on another hierarchy lever, the previous selection will be cleared first, then new file would be marked.
It's often useful to mark all files in a folder.
This may be achieved by using the check mark of the parent folder. First click on the check mark will mark the folder itself. Second click on check mark of already marked folder will mark all files contained in the folder (this assumes that the folder is expanded and files inside are currently shown). Third mark on a folder (when there are already marked some of its child files) will clear entire selection.
Marking all files may be useful also for case when you want to make operation (copy/delete/etc) on all files in a folder except of some. You'd first mark all, then unmark few of them that you don't want to work with.
You may also mark range of files in a folder. That means marking continuous range of files as they are currently sorted and displayed.
To achieve this, click on check mark of first file in desired range. Then long-click on check mark of last file in range. All files between marked files will be marked too, and you can work with this selection.