It depends. If the text is listing the stars of the show, the producer, director, etc., it is called "credits." If it is text that is translation the spoken dialog, it is called "subtitles." If it is written text of what is being said onscreen (to help the hearing-impaired, for example), it is called "closed captioning." If it's something like a descriptive sentence or two (for example, a tornado warning from the National Weather Service), it is called a "text scroll."