When you get an app, it's in an apk file. That's actually a zip file. It has the app, any files that the app needs (pictures, spreadsheets, data, etc.) And instructions to the installer as to how to install the app - what goes where.
Package installer is the program that opens the apk file, reads the instructions and installs the app.
If you were wondering because you were thinking of uninstalling it to gain some space, don't. All you'll "save" is the ability to install new apps.