There used to be dos programs and programs that needed windows, later dos programs referred more to things that used the command line and looked like dos programs of old rather than using Windows based methods to display itself.
When dos started to die out as a primary means of using a computer various programs were converted to play to windows standards, usually by getting the source code and making it instead use Windows methods for displaying programs aka a port. This is what that is.
It is and always was a general purpose text decoder program. If the game (or whatever file you are feeding it) falls within the standards it works to* then you will be able to display and do limited editing to things. Could be a DOS game, could be a NES game, could be a GBA game, could be an arcade game... it matters little.
*I am not sure offhand of Thingy32's ultimate limitations but there are probably some really exotic modern PC games, most likely from Asia, that use a format it would struggle with. Anything that is normally looked at on this site will probably be fine though.
How many people would suggest learning it today, or only use it today because that is what they know, I am not sure about. Despite being a port of an older program it is still a bit long in the tooth with its share of bugs. To that end many would probably suggest you find something a bit more modern, but if you must there are worse choices.