What does it mean by P:N ratio of a CMOS inverter with equal rise and fall times?
The PMOS transistor has a lower driving strength then the NMOS transistor. For equal rise and fall times the PMOS transistor needs to have a larger width than the NMOS transistor.

Typically the PMOS transistor is about 3 times wider than the NMOS transistor.

FO is the fan-out, which is the number of gates that can be driven by that inverter.