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How do bromine atoms help propagate the reaction between hydrogen and bromine?

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dissociation of E–N — reaction mechanism Possible energy diagram for the dissociation of a covalent molecule, E–N, into its ions E⁺ and N⁻ (see text).

In the reaction between hydrogen and bromine, bromine atoms act as essential intermediates in a chain reaction. The process is propagated through a cycle of two specific steps that consume and regenerate bromine atoms, allowing the reaction to repeat multiple times:1

Step 1: A bromine atom reacts with a hydrogen molecule to produce hydrogen bromide and a hydrogen atom: $Br + H_{2} \to HBr + H$ .1
Step 2: The resulting hydrogen atom reacts with a bromine molecule to produce another molecule of hydrogen bromide and regenerate the bromine atom: $H + {Br}_{2} \to HBr + Br$ .1

Because the bromine atom consumed in the first step is regenerated in the second, this pair of reactions can occur repeatedly without a net loss of bromine atoms. These two steps are known as the chain-propagating steps, and the number of times this cycle repeats is referred to as the chain length.1