Teachers are supposed to have 10% of the teaching timetable (generally around 25 hours a week) when they are not teaching - this is called 'PPA'.
But most FT teachers work anything between 50 and 70 hours a week, so the time they spend doing their planning, marking, responding to parents, meetings with outside agencies, meeting with colleagues, admin, tracking, and general pointless paperwork, is more than the amount of time spent teaching.
The comment was in reply to Sadcafe asking how teachers could work from home, suggesting that the idea of people taking their PPA time off site, was a bit ridiculous. When clearly lots of this work is done at home already - evenings and weekends. Nobody questions all of those hours being done at home.