Open the file manager on your Android phone. Whatever it can be, the default one or any other file manager that you are using. In my case, I'm using the one in the above screenshot.
On your file manager, choose the folder that you wish to hide. In my case, I have chosen the folder with the name "Test folder" for this purpose.
Select the folder by long pressing on this folder. When the folder is selected, you will get to see many options. From there you will find the "Rename" option.
Now, you have to put a dot (.) before the name of the folder and tap on OK. In my case, I've put a dot before "Test folder" and now the new name is ".Test folder".
Your phone will ask you to confirm that process with a note that it will hide after this change. Tap on OK to confirm the changes.
Now you have successfully made your folder to hide. To review the folder, open "Show hidden files". Tap on it and there you will see your folder.