The vo'cume projector, also spelled vocuume, was a device of Goa'uld design.
These machines were small handheld devices that were roughly cylindrical in form. Internally, they contained a holographic projector making it capable of displaying a large three-dimensional image which was pre-recorded.
The Goa'uld used the vo'cume as a way of summoning troops, giving orders, authorizing spokespersons or dispensing other messages to the Jaffa legions. The use of this device during battlefield operations was strictly forbidden under the rule of engagement.
Major Samantha Carter managed to interface with this vo'cume and use its holographic technology to display the final moments of Apophis before his death when Sokar attacked Stargate Command for the former System Lord. This resulted in his followers realizing that the Goa'uld was not a god, which ended the hostilities towards SG-1. (SG1: "Rules of Engagement")