If you want to see all hidden files in a folder, go to that folder and either click View ▸ Show Hidden Files or press Ctrl+H. All hidden files will be shown along with the files that were not hidden.

Some hidden files will have a "." at the beginning of their name. Others might have a "~" at the end of their name instead (see What is a file with a "~" at the end of its name?).

To hide these files again, either click View ▸ Show Hidden Files or press Ctrl+H again.

Hidden files will only be shown in the current window, until you turn the option for that window off. To show hidden files in all file manager windows, see File manager views preferences.

To unhide a file, go to the folder containing the hidden file and click View ▸ Show Hidden Files. Then, find the hidden file and rename it so that it doesn't have a "." in front of its name.

For example, to unhide a file called .example.txt, you should rename it to example.txt.

Once you have renamed the file, you can either click View ▸ Show Hidden Files or press Ctrl+H to hide any other hidden files again.