The product of a number and its reciprocal equals 1.
The reciprocal of 10 is 1/10.
The reciprocal of 3/4 is 4/3. (i.e. you invert or swap the numerator and denominator)
The reciprocal of 1 is 1.
The number 0 does not have a reciprocal because the product of any number and 0 always equals 0.