The step of chain initiation produces a Br radical from HBr.

On the first step of chain propagation, the Br radical forms a C-Br bond, for which the second electron is pulled from the C=C bond, which breaks, leaving the second electron on carbon (carbon-centered free radical).

On the second step of chain propagation, the carbon-centered free radical abstracts the hydrogen atom from HBr, generating a Br radical, which goes 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.