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The term free streaming refers to the movement of particles over long distances without any absorption or scattering over that distance. The free streaming scale (or free streaming length) of some type or classification of particles is a general distance these particles do this, the term used for measurements, analysis, or estimates. (Such a free streaming scale can be worked out from the number density and cross section of particles that could interact with the streaming particles). In cosmology, those particles would be declared to be free streaming if their free streaming scale is a cosmological distance.
Photons from the Sun reaching Earth are free streaming over that distance, whereas deep within the Sun, they are absorbed, re-emitted and otherwise scattered, and when they reach Earth atmosphere, some are scattered, producing the brightness of the sky. Neutrinos free stream from the Sun's core, generally right through Earth because their interaction with other matter is so rare. The photons of the CMB that we observe have been free streaming from the recombination.