An en bloc clip is a means of storing and loading ammunition into a rifle (I don't think there are any pistols that use en bloc clips). Unlike pistols, which generally use removable magazines if they are autoloading, some rifles have built-in magazines. These are fixed spaces inside the rifle where bullets are inserted, and which usually contain mechanisms to feed the rounds into the action. On some of these rifles, rounds are loaded individually directly into the magazine. Some use what are called stripper clips. Some use en bloc clips.
It is essentially a formed piece of light metal that holds a number of cartridges in the proper orientation to be inserted into the (usually spring-loaded) magazine. Unlike stripper clips, en blocs are loaded into the gun with the cartridges, and are ejected when they are empty either manually or automatically. In the case of the M1 Garand, the associated en bloc clip looks like a small cup of metal into which the base ends of eight .30-06 Springfield rounds are inserted in two staggered rows of four. The entire clip is pushed down into the magazine.
The en bloc clip was invented for the Lee Rifle by James Paris Lee in the late 1800s. Although Lee was a proponent of the removable magazine, at the time they were very expensive to manufacture which meant that the stamped-metal clips could be made in much larger quantity. During World War II, en bloc clips were mostly treated as disposable, and ammunition was shipped to the front already loaded into en blocs. The en bloc was also simultaneously invented by Ferdinand Mannlicher for use in his own repeating rifle designs (Lee would sue Mannlicher in 1891 for patent infringement over this, unsuccessfully).