Can someone explain to me the multiplicative inverse in detail i don't understand

For exemple the multiplicative inverse of 1/2 is 2/1 <=>2 The reciprocal for a number x is 1/x

Well first you need to look at the definition which happens to be 

: Also known as a reciprocal for a number x, detonated by 1/x or x-1 , is a number in which when it is multiplied by x yields the multiplicative identity .

for example the reportorial for the number 5 is one fifth = 1/5 also know as 2.0
and the reportorial for 0.25 is i / (divided by) 0.25 or even 4
 
now that you have read the meaning you should have a better understanding

i hope this helps