A number to the power of negative one is equal to one over that number. For example, five to the negative one power equals one over five, or 1/5.
The negative exponent rule says that when doing calculations with negative exponents, any negative exponents in the top half of a fraction get moved to the bottom half and become positive exponents. This works for any negative power, not only the power of one. For example, to calculate five to the negative two power, first move the expression to the denominator and make it positive. This gives one over five to the positive two power, which is equal to 1/25.
The reverse is also true. Negative exponents in the denominator of a fraction get moved up to the numerator and become positive exponents.