Runtime Package is a feature introduced in RPG Maker 2000. It was introduced to reduce the file size of RPG Maker games, by eliminating the requirement for the default RPG Maker assets to be included in the game. Instead, the game player will install a default package, which the game will rely on when using the default pre-made assets.
However, the feature is dropped in RPG Maker MV, which includes all used pre-made assets in the output versions of games. RPG Maker MZ also functions in this way.