nmr spectrum:

The signals at the right side of an nmr spectrum are because of protons comprising a low precessional frequency whereas the signals at the left-hand side are because of protons having a high precessional frequency. Low precessional frequency is related with high electron density (shielding) whereas high precessional frequency is related with low electron density (deshielding). The energy needed for resonance increases from right to left.

Figure:   An nmr spectrum.

 A scale is required in order to quantify the position of signals in an nmr spectrum. The scale employed in nmr does not have absolute values, but is relative to the signal of a reference compound termed as tetramethylsilane (TMS). The methyl protons of TMS are equal and give one signal that is described as the zero point on the scale. The scale is termed as the chemical shift and is calculated in parts per million (ppm).