Scientific notation writes a number as a × 10n, where a is at least 1 and less than 10. Move the decimal point until only one digit is left in front of it; the number of places you moved it is the exponent, positive if you moved left and negative if you moved right. Engineering notation is similar but keeps the exponent a multiple of 3.
123,000 = 1.23 × 105. The decimal point moves 5 places to the left.
0.00045 = 4.5 × 10-4. The decimal point moves 4 places to the right, so the exponent is negative.
It is E-notation, the way calculators and programming languages write 1.2 × 106, which is 1,200,000.
See also: Decimal to Fraction Converter · Fraction Calculator · Binary to Decimal Converter