The Satchel Charge is an explosive demolition bag featured in Half-Life and its expansions. It is more destructive than the Mk 2 Grenade and can be detonated from afar by a remote-controlled detonator, much like the S.L.A.M.
Satchel Charges are basically very large grenades, or the equivalent of a C4 charge.
As they are larger and heavier than hand grenades, they cannot be thrown as far: the result will however be a larger explosion.
Pressing the primary fire button drops a single Satchel Charge, then the detonator will be automatically equipped. One can then either detonate the Satchel Charge with the primary fire button or drop additional Satchels with the secondary fire button. If multiple Satchels have been dropped, pressing the detonator will detonate all of them.
At some point in the Topside Motorpool, Freeman is crawling in a duct and a HECU soldier opens a hatch at the end to throw a Satchel Charge in. To survive, Freeman must quickly go back at the end of the duct and dive into the water when flames engulf the whole duct. However, while the latter is the effect intended by the developers, the HECU soldier can be killed before he throws the Satchel Charge.