Remote detonator, or timer
C-4 is a powerful high-velocity plastic explosive created by the Tau'ri.
They are usually contained in block form, though the plastic itself can be molded into other shapes. (ATL: "Quarantine") The explosive is usually detonated if a detonator is placed into the C-4 and remotely activated, or placed on a timer. The destructive power of properly placed C-4 has proven very effective in fighting the enemies the Tau'ri has faced. Placing the blocks in key areas can destroy a Goa'uld Ha'tak vessel and even Wraith Hive ships, as well as blow open doors and destroy other key enemy targets. However, they are ineffective against Kull warrior armor and are somewhat ineffective to Tau'ri blast doors unless used with a second pack. C-4 is also used in A-M18A1 Claymore mines. (ATL: "The Last Man", "Broken Ties", SG1: "Within the Serpent's Grasp", "The Serpent's Lair", "Evolution, Part 1", "Reckoning, Part 2")
(The emphasis on C-4 being the single important thing the Genii would need to make their weapons is a simplification. A compressive explosive lens would be much more difficult to design -especially timing the many detonators required-, so simply having the plastic explosive would help only a little compared to the technical specifics regarding the design).
During the Battle of Atlantis, the Expedition were able to trade C-4 with their experimental nuclear weapons.
SG-1 also destroyed two Ha'tak's: C-4 charges crippled one and apparently it lost control and crashed into the second, or the final explosion destroyed both. (SG1: "The Serpent's Lair") (ATL: "Underground", "The Storm", "The Siege, Part 2")
Bra'tac called C-4 4-C until he was corrected