Yes, it's like a supply teacher. Many schools use cover supervisors these days, as it's more cost effective than getting in supply, and as you'd be on the staff full time, you have more opportunity to build a rapport with classes.
You should be left work with clear instructions for each class, and would be responsible for monitoring behaviour and work in the lesson and reporting back to the teacher, though not responsible for planning or assessing work. There would be periods of time when you might not be needed for cover, and the school should redirect you to something else during this time - some schools fill up free slots with admin, or pupil support.
It is often a starting point for people considering teaching, as it's a really helpful thing to put on your cv, and getting onto teaching postgrad courses is really hard these days, so experience like this counts for a lot.
The upsides are that if you want a term time only, short working day, it's about the only thing available, other than teaching assistant. You don't have to start til 8.30 and as you say, you can walk out at 3 with no meetings, marking or other responsibilities.
The downsides are that the money is usually poor, and if the school is a tricky one, you may get a lot of aggravation from pupils, who can tend to be difficult when their usual teacher is away.
But at the end of the day, if you can keep reasonable order, and get them to produce the work set, your responsibility ends there - you aren't going to be measured on how they do in exams etc.
HTH!