To distinguish different tissues, we need to obtain contrast between them. Contrast is due to differences in the MR signal, which depend on the T1, T2 and proton density of the tissues and sequence parameters.
The higher the signal is, the brighter it will appear on the MR image. Interpretation is based on analysis of tissue contrast, for given signal weightings (T1, T2, T2* or PD).
MR image could be compared to the representation of a painting with only 2 colors. For example, red would correspond to the T1 effect, yellow to the T2 effect, and pigment density to proton density. If we change the TR and TE, we can see better the red or yellow part of the painting better.