On the step of chain initiation, Br free radicals are produced by cleavage of the Br-Br bond upon heating or irradiation with light.
On the rate-limiting first step of chain propagation, the Br radical abstracts a H atom from carbon, producing a free radical and HBr.
On the second step of chain propagation, the carbon-centered free radical abstracts Br from Br2, generating the final organic bromide and a Br radical, which proceeds back to the first step of chain propagation.
On the step of chain termination, any two free radicals bond together for the account of one unpaired electron from each radical.