EFP is an acronym meaning: Explosive Formed Projectile.
An explosive formed projetile is simply that, a projetile that is formed by an explosion that is so powerful it can penatrate many layers of armor before it ever begins to slow down.
How it works is an explosive is packed into a tubular or cylendrical case and the open end is capped off with a cone or dome shaped sheet of metal (typically copper). When the explosive is detonated the metal end cap is super heated and melted into a molten projectile as it is propelled forward at astronomical speeds (measured in miles per second) by the explosion. When the projectile then comes in contact with the target it is still super heated as it melts through the target and at the same time its inertia continues to drive the projectile further through the intended target.
EFPs are not new to warfare technology. The common RPG
carried by insurgents in Iraq and Afgan are EFPs but on a smaller scale and are just built into a projectile. Also some anti tank rounds and shape charges use the same simple concept.