Essentially two weapons joined together, in the case of bolter-flamers and bolter-meltaguns, combi-weapons give the model a choice of two weapons to fire. A Chaos Space Marine armed with a bolter-flamer or bolter-meltagun may choose to fire one of the weapons 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 the same time.
Combi-weapons – Bolter-flamer & Bolter-meltagun
Source: Codex: Chaos Space Marines (1999), p. 20