'RTF' stands for 'Rich Text Format'. It's a file-format developed by Microsoft to allow for cleaner transfers of documents from one word-processing program to another. [Rich Text Format @ Wikipedia]

We're asking that all submissions be in RTF to ensure that we can get your work to our readers in a form that's easy for them to work with, and that we can get manuscripts that have been selected to our editor and designer in a form that they can readily use. (Which helps you by ensuring that our readers, editor and designer don't have to work hard to read your work.)

Most modern word processing software can save files in RTF format. [Hints on how to convert your document to RTF.]