In the early 1890s it was realized that en bloc clip loading systems in bolt action military rifles was not ideal. It meant that a clip was relied upon to allow the gun to function as a repeating rifle as it served the function that a modern magazine would. The open hole for the clip to fall out was susceptible to dirt and grit jamming the action and that was not good for combat use.
The Mauser brothers came up with a solution. It involves a conventional magazine on the gun like we would see today on many bolt action rifles of both sporting and military variety in which the magazine is enclosed at the bottom and features a leaf spring that pushes on a flat piece of metal called a follower that propels the rounds up. Spring loaded bars on the sides of the magazine allow for reloading and controlling the feed of bullets one round at a time. But what changed things was the stripper clip or charger they devised.
Nearly all military rifles and some pistols of the late 19th century clear up to the post World War II era use this system to reload rapidly but today it is mostly used to rapidly reload a detachable magazine for use later. Nomatter how it is used the clip itself does not seat into the rifle but only holds the rounds as a unit and falls free after the action of loading. In a rifle or early automatic pistol it involves a notch milled above the magazine so when the bolt is pulled back the clip is seated into the notch and the user puts his or her thumb over the rounds and pushes them down into the magazine with the clip staying in place. All that is required now is to push the bolt back forward and the clip falls free. You are not reloaded. The same is done on empty magazines off a modern rifle when loading each round into the magazine would be a real chore. It takes a split second to accomplish it with a stripper clip.
The clips are cheap and reusable but are also specific to each gun model. Like the en bloc clips they are made of spring steel and rely on spring tension to keep the rounds together. Very old clips may be weak to use but for the most part they are very reliable. Another easy way to reload your gun quickly.