Imagine running: running freely in an open field (air) is very fast, running through waist-deep water slows you down (water), and running through thick mud is extremely slow (glass). The denser the medium, the higher the resistance to light (higher refractive index).

AIR is fast (low n), WATER is moderate, GLASS is dense (highest n): $1.0 < 1.33 < 1.52$.

Refractive index $n$ is defined as the ratio of the speed of light in a vacuum ($c$) to the speed of light in the medium ($v$), given by $n = \frac{c}{v}$. Since light travels fastest in air ($v \approx c$), $n_1 \approx 1.0003$. In water, light slows down ($n_2 \approx 1.33$), and in crown glass, it slows down even further ($n_3 \approx 1.52$). Thus, $n_1 < n_2 < n_3$.

Remember that optical density is different from mass density; turpentine has a lower mass density than water but a higher optical density (higher refractive index).