A rocket-propelled grenade (RPG) is a shoulder-fired, anti-tank weapon system which fires rockets equipped with an explosive warhead. These warheads are affixed to a rocket motor and stabilized in flight with fins. Some types of RPG are reloadable while others are single-use. RPGs, with the exception of self-contained versions, are loaded from the muzzle

Muzzleloader

A muzzleloader is any firearm into which the projectile and usually the propellant charge is loaded from the muzzle of the gun . This is distinct from the more popular modern designs of breech-loading firearms...

. RPGs are very effective against armored vehicles such as armored personnel carriers (APCs).
In the context of "rocket-propelled grenades", RPG is a transliteration of the Russian РПГ or ручной противотанковый гранатомёт (transliterated

Scientific transliteration

Scientific transliteration, variously called academic, linguistic, or scholarly transliteration, is an international system for transliteration of text from the Cyrillic to the Latin alphabet...

 as "ruchnoy protivotankovy granatomyot"), which translates to the English phrase "hand-held anti-tank grenade launcher".