There are typically 3 possible numbers for quantitative spectral data.
Digital Number
Digital number is the value as stored onboard the sensor, typically an 8, 12, or 16-bit value.
Radiance
Radiance is the quantitative strength of the signal. It can be thought of as the brightness, but as brightness depends on distance, it is measured per spherical angle to compensate, e.g. as watts per meters squared per steridian (sometimes as watts per meters squared per steridian per nanometer when dealing with hyperspectral data).
Reflectance
With DN as the stored value, and radiance as the quantized value in measureable units, reflectance would be the ratio of reflected light and incident light for a given spectra. So it can be thought of as the inherent brightness of the object in a given spectrum, independent of e.g. how bright the sun is shining.
In a way, reflectance is the most like the concept of color, or color as independent from lighting anyway. As such, it is useful to use in indices like NDVI because it represents the object independent color.
(All that said, the model is complicated by the fact that in addition to reflectance, we have scattering, absorption, and fluorescence (at least in plants). So, in certain spectra, the apparent reflectance values can be increased by absorbing in one frequency to be fluorescence in another frequency.)
As a ratio, it should be a value between 0-1.