I can't believe that someone is actually asking this question. Text files (denoted with a txt extension) are plain text files, with no formatting, i.e. no bold or italics. If a piece of software comes with a readme file, it is usually in a txt format. Presuming that you are using Windows, you can open and save text files with Notepad, which is on every version of Windows. Word can also save documents in txt format as well if you use the Save As option as can WordPad. It is the most common text format you can have on a computer.
Text files cannot contain pictures so you cannot have anything other than plain text in them. You can store any file on your J3 when you drag and drop or copy and paste a file, but only text files can be read by the J3 itself. If you plug your J3 into another PC it should show up as an external drive, so you can copy the file to the PC as if you were using a flash drive. The J3's primary function is to play music.