Multiplying a decimal value by 10 raised to a power is equivalent to moving the decimal point to the left or right the number of times indicated by the power.
In the case of a negative exponent, the decimal is moved to the left (this is the same as dividing by 10 a number of times).
In the case of a positive exponent, the decimal is moved to the right (this is the same as multiplying by 10 a number of times).
The negative exponent here, −4, indicates that the decimal point is to be moved to the left 4 places, \([6.6 \times 10^{-4} ] \) = 0.00066.