Teacher talking time (TTT) is the time that teachers spend talking in class, rather than learners. It can be compared with, student-talking time. One key element of many modern approaches is to reduce the amount of TTT as much as possible, to allow learners opportunities to speak, and learn from speaking.
A teacher monitoring students working in groups completing a discussion will probably do fairly little talking, limiting themselves to clarification of the task and offering language when requested. The same teacher leading an inductive grammar presentation will probably talk more, as they explain, illustrate and check understanding.
In the classroom
The relative value of TTT and STT is a complex area. Learners need to produce language in real time conversation; to give them a chance to notice their own mistakes, and for the class to be student-centred. They also need input from an effective language user in order to form hypotheses about language rules, and the teacher may be one of the main sources of this input. A teacher can start exploring this area by taping themselves and finding out when, why and how much they and their students talk.