These are basically two weapons joined together, giving the Space Marine a choice of two weapons to fire instead of one. A Space Marine who is armed with a combi-weapon may choose which of the weapons he is going to use in the shooting phase. The bolter may be fired any number of times, but the other weapon may only be fired once per battle. Note that you may not choose to fire both weapons at once.
Errata — Chapter Approved: Codex: Space Marines Q&A (v3.0): The Codex: Space Marines combi-weapon rule is intended to eventually apply to all combi-weapons army-wide, but for the time being use the combi-weapon rule from the codex that the army was picked from.