They promised to scrap the tuition fees introduced by Tony Blair's Labour government and then increased by Labour after the party had clearly and explicitly promised in their 2001 manifesto not to do so.
Jeremy Corbyn also referred to the existing debt owned by former students who went to university since fees were introduced and clearly implied that he would write off that debt.
A good example of what Labour was saying during the election can be found in an interview he gave in the NME dated 1st June 2017 which can be found on their website under the title
Jeremy Corbyn: "I will deal with those already burdened with student debt."
Let me quote verbatim the relevant part of the interview:
Labour still pretend that they intend to scrap tuition fees for future students but have now more or less admitted that the idea of writing off the debts of past graduates is just an "ambition" which they are not going to promise because they don't want to promise things they cannot actually do.
But as Henry Hill writes on Conservative Home here, just as Corbyn's comments about how he wanted to write off student debt cut through to many people, not all of whom would normally have considered voting Labour, a lot of those people have noticed that Labour has now rowed back from those election campaign comments.
Ask Nick Clegg or any of the other Lib/Dems who lost their seats as MPs or councillors between his student fees U-turn and this year what happens when your promises run too far above your ability to deliver.
One of my colleagues suggests that every time Corbyn or one of his team makes an unrealistic or undeliverable promise from now on we should all call out "free unicorns."
I suspect there may yet be plenty of opportunities to do so.