From the app's main interface you can choose the shortcut you want to use (by default, Control+Shift+F for closed folders). You can also see a list of recently closed applications and folders.
This way, you can quickly reopen a window, no matter whether you closed it by accident when you were clicking too fast, or just closed it intentionally and need it open again. As soon as you press the keyboard shortcut the last window or folder you closed will reopen.
UndoClose is a very useful tool that barely takes space on the memory, runs discreetly, and can be integrated into Windows. Likely to become another of your go-to tools on Windows.