I don't fully understand Corbyn's stance on this...
It is very straightforward. As part of the EU, there are agreements as to how workers should be treated. This is essential if you are to have a level playing field.
Corbyn suspects - and I'm sure he's right - that things like a minimum paid holiday entitlement, sick pay, a degree of job security etc would rapidly be abolished by this government were we out of it. This would give them a cost advantage over the EU we would be able to undercut them - at the cost of poorer working conditions. The unions have already been weakened by successive governments, and no doubt would face further legislation.
Incidentally, the EU has no power to make laws over us unless we - as a country - have already agreed to them being tabled for approval by the European parliament. If we don't agree with them we can stop them. It is perfectly democratic.
I suspect that what happens is that our government agrees to something it doesn't like in exchange for getting some other piece of legislation through. Horse trading in effect.It is very convenient to be able to blame the EU for the stuff they don't like, particularly if people are ignorant of how it works.
I think the Media are to blame for this. Most folk know more about american politics than about European politics which makes no sense to me whatsoever.
However you paint it, leaving is a huge economic risk. We currently export over a million cars a year into Europe and the rest of the world, because many foreign manufacturers - mainly from Japan - see us as a convenient place within the EU from where they can ship cars across the continent without fuss. If we leave, I am sure companies like Nissan will too - probably to countries where labour is cheaper, but within the EU.
As an island we are prone to xenophobia and suffer - still - from a 'them and us' attitude to the EU. This is not true of the european community in general where driving into a neighbouring country every day to work is quite common.
You can see what the financial industry thinks of leaving by the slump in the pound and the share prices when News
of the Independent's poll giving the Leave side a 10 point advantage yesterday. These are not folk you would trust, necessarily, but they probably gave a better idea of the financial shock awaiting us than most.