What does the farad rating mean in capacitors?

The rating or 'size' of a capacitor, called its "capacitance", is related the amount of

charge the capacitor can store, to the amount of energy it holds when it stores some

charge, and to the opposition of the capacitor to the apparent flow of alternating

current through it.

If a capacitor has a capacitance of 1 farad, then

-- One coulomb of charge stripped off of one plate and added to the other plate produces

1 volt of potential difference between the plates.

-- The energy stored in the capacitor is 1/2 the square of the voltage across it.

-- Its impedance is (0.159 divided by the frequency) ohms.

The farad is an enormous capacitance. A typical capacitor used in a 'lumped' circuit ...

the kind of construction where you would buy a capacitor and solder it in ... has a

capacitance in the range of maybe 10 picofarads (trillionths of a farad) to maybe

100 microfarads (millionths of a farad).