Here is a prefix in use:
e.g. A 2·3 kW kettle is connected to the 230 V mains. What current flows through its heating coil?
P = I × V
So I = P / V
I = 2,300 / 230
I = 10 amps
2·3 kW needs changing into 2,300 W.
If we had left the power as 2·3, our answer would have been 0.01 A - way too small for a kettle!
Whenever you see a prefix in front of a unit, remember to change the number by the correct amount!