What does the PAM4 level value represents?
This could be anything really. 

You would classically have a 'high level' and a 'low level' in NRZ, single ended links. A '1' will be represented by you 'high' level, and a '0' will be represented by your low level. Perhaps this is 0V and 1V respectively. 
But in PAM4 you need 4 levels. In this case they achieve this by using both positive and negative levels, so -3, -1, 1, and 3. The difference between each is 2. 

This just means some arbitrary unit. It could be that you represent the signal in current, and that the number is in mA (so we have -3 mA, -1 mA, 1 mA and 3 mA). But it could also mean voltage and be unrelated by the actual number, so the -3 could be 200 mV, -1 could be 400 mV, 1 could be 600 mV and 3 could be 800 mV.