The signum is a special form of communication device that allows the Techmarine to access a myriad of useful battlefield targeting information, and then pass it on to his fellow battle brothers. In game terms it allows you, each turn, to re-roll one missed to hit shooting roll for the Command squad that the Techmarine belongs to.
Errata — Chapter Approved: Codex: Space Marines Q&A (v3.0): The re-roll convention is that a re-roll must be used before any further dice are rolled — you cannot wait until the whole squad has fired (or rolled to wound, etc.) and then decide which die to use the Signum's re-roll on.