If light enters matter, its properties may change. Namely, its intensity (amplitude), polarization, velocity, wavelength, etc. may alter. The two basic phenomena of the interaction of light and matter are absorption (or extinction) and a decrease in velocity.
Absorption means that the intensity (amplitude) of light decreases in matter because matter absorbs a part of the light. (Intensity is the square of amplitude.)
The decrease in velocity (i.e. the slowdown) of light in matter is caused by the fact that all materials (even materials that do not absorb light at all) have a refraction index, which means that the velocity of light is smaller in them than in vacuum. The refraction index is the ratio of the velocities of light measured in vacuum and in the given material.
The following animations present these basic phenomena.