What is coupling constant J?

Hint :We know that the coupling reaction refers to the class of organic reactions that involve the joining of two chemical species usually with the help of a metal catalyst i.e., it is a reaction in which two fragments are joined together with the aid of a metal catalyst.

Complete Step By Step Answer:
As we know, the coupling constant is usually denoted by J and it is used to measure the interaction between a pair of protons in an atom. It is basically used to measure the interaction or magnitude of splitting effect and it is represented by ‘J’ having units of frequency \[\left( Hz \right).\] Coupling constant(j) depends only upon the molecular structure of the compound and independent of applied field strength unlike the chemical shift difference in parts per million (ppm) between the resonance frequency of the observed proton.
For a pair of mutually coupled protons, the coupling constant due to splitting by one proton has the same value as the coupling constant due to the second proton. Value of coupling constant(j) varies from $\left( 0-20 \right)Hz$ for the proton attached to the same carbon atom (i.e., germinal protons) and depending upon the bond angle and molecular structure of the compound. The coupling constant J is pretty much the peak-to-peak distance, usually reported in \[Hz.\] Matching it up with other nearly-identical coupling constants elsewhere in the spectrum usually tells you which protons are near which others.

Note :
Remember that the simple case of doublet, coupling constant is the difference between two peaks and it is not calculated in ppm. And if you want to calculate the coupling constant then you have to just take the difference of two frequencies which you get by NMR machine.