The center of a multiplet is the chemical shift - either the middle peak (if it is an odd number: singlet, triplet, quintet, etc.) or the average of the two middle peaks (if it is an even number: doublet, quartet, sextet, etc.). It is reported in ppm.
The coupling constant is the distance between any two adjacent peaks in the multiplet - they should all be the same. Coupling constants are reported in Hz.
The reason is that the chemical shift depends on the external magnetic field, while the coupling constant depends only on the magnetic field generated by the individual nuclei. This means that the actual energy of the chemical shift (in Hz) will change relative to the size of the magnet, while the coupling constant will not. As the magnet size gets larger, the center of the multiplets will remain in about the same place, while the peaks of the multiplets will appear to get closer together, because the number of Hz in the coupling constant is fewer ppm.