The Payment Threshold is the minimum you need to earn before the Income Share Agreement kicks in. If you earn below the Payment Threshold you do not have to make payments towards you Income Share Agreement. As this is a graduate programme we expect the Payment Threshold to reflect your higher level of education and thus earning ability.
Some companies also call the payment threshold the 'earning threshold,' 'earnings threshold' or 'salary floor'. They are all the same thing. Occasionally, we may slip up and also call it the earning(s) threshold.