A long time ago, in the Bronze Age, there were a group of people living in Scandanavia. These people would eventually become the Germanic people. The people living then didn’t really have writing. They had runes but they weren’t used much. They were mostly used for ceremonial purposes like on swords or other blades.
The fact that these people didn’t write many things down means that we know very little about their day to day language. They obviously spoke a language, and we can make some very educated guesses as to their language but we have very few things that directly indicate what their language was like. We don’t know what their language was called, so linguists decided to call it Proto Germanic because it is the language all Germanic languages come from.