A frame is a set of meaningful signs connected to specific types of social action. Together they create a 'logic' of action and make the action understandable for those involved in it.
In Frame Analysis, Erving Goffman developed the notion of 'frame' as a key to understand how human beings organize their social experiences (the subtitle of the book is An Essay on the Organization of Experience). We do that, argues Goffman, by producing signs that point towards particular types of activities. For instance, the start of the frame of a 'class' can be signaled by the lecturer entering the lecture room, opening his/her didactic tools (texts, slides) and announcing the start of the lecture with a greeting or with something such as 'have you done your readings for today?' The end of the class frame will be likewise signaled by the lecturer saying something like 'okay class, thank you, see you next Thursday and don't forget to do your readings'. In between both the starting signal and the ending one, the entire set of activities will be framed as 'a class' and every aspect of conduct will be evaluated in terms of the norms perceived (or imposed) as valid for and within a 'class'.
Goffman was inspired in his development of 'frame' by Gregory Bateson's notion of 'metacommunication'. Bateson had observed animals playing, and had noticed that 'playful' fight, as opposed to real, hostile fight, was flagged by animals through a set of very small and delicate signals 'framing' the action as 'play'. Goffman took Bateson's fundamental insight and took it further, infused and enriched by his own 'dramaturgical' perspective on human interaction. Goffman would emphasize the orderly nature of human interaction, but also the fact that such interaction orders had to be performed actively by participants. Frames were among the key devices by means of which we arrive at such performed orderliness.
Frames within frames
Goffman distinguished between different types of frames, ranging from general to very specific. This means that in the same social event - take, for instance, a class - several frames can be simultaneously at play. The general frame of the class can be combined, for instance, with several subframes such as group-work, discussion, Q&A sequences, interruptions or so-called 'frame breaks' (e.g. when someone's smartphone goes off), and so on. This means that, in any analysis, one should look not just for the major frame laying out the main scenario for the event, but also for the several subframes occurring within it. Social events are rarely homogeneous and most often 'composite', multiframe events.