PIC24EP: What exactly are IOL, IOH currents
Why is the IOH limit is mentioned negative?


It's just a convention thing. Normally currents into the device (supply currents etc.) are attributed positive values therefore, currents from chip pins are given a negative sign.


  What do the IOL, IOH currents exactly represent? Are these the
  sourcing / sinking currents



IOL is the current into the pin (sinking) when the pin has a logic level of zero.
IOH is the current from the pin (sourcing) when the pin has a logic level of one.


All the data in the table in the question refer to the pins being outputs (3rd column of table).