STT means Student Talking Time. It is the time learners spend talking rather than the teacher. It can be compared with Teacher Talking Time (TTT). It can be a useful category for observation of teaching, or for self-reflection about teaching.
Group work with the teacher monitoring, rather than an open class, is a way to increase the STT in an activity.
In the classroom
Many teachers (and approaches) seek to maximise the amount of time learners spend using the target language. One way to do this is to ensure that learners know the necessary classroom language, e.g. to ask for clarification and express doubts.