I would think it is almost certain that this is a wall-mounting plate bearing a socket outlet (usually, co-axial type) suitable for TV or satellite aerial (antenna) connections and also cable TV, of course.

'Jack' is a US word for what we in the UK are more likely to call a 'socket' (except in certain specific contexts) --- it's a bit confusing, 'cos in UK English we talk about a 'jack' most often in reference to a particular type of (cable-mounting) plug known as a 'jack plug' which mates with a 'jack socket', which in AE would be a pleonasm!