I - current in Amps. The I comes from the archaic term intensity.

Isc - Amps when short circuited; this is when your solar panels are connected only to themselves. This is the highest current your panels can theoretically produce.

V - Volts

Vabs - charge controller voltage setpoint for Absorption stage

Vfloat - charge controller voltage setpoint for Float stage

Voc - volts in open circuit; when your solar panels are not connected to anything. This is the highest voltage your panels can theoretically produce.

W - power in Watts