FOSC is the frequency of the external oscillator/system clock (e.g. 16MHz)
TOSC is the time for one FOSC oscillation (i.e. 1/FOSC) (e.g. 1/16MHz = 125nS).
FCY is the frequency of the instruction clock (i.e. FOSC/2) (e.g. 16MHz/2 = 8MHz).
TCY is the time for one instruction cycle (i.e. TOSC/2) (e.g. 125nS/2 = 62.5nS).