Proto-Germanic is, technically speaking, not a real language itself. It's a theoretical ancestor. The word "proto" indicates this theoretical nature. Think of proto-Germanic maybe as a long lost ancestor in your direct family line; that ancestor must have existed and have possesed at least some genetic markers (or other traits) that are likely to have been passed down to you, but you don't have any photographs, names, census records, ... Nothing. You simply know that this ancestor existed and that you can make educated guesses about this ancestor if you look at everyone in your famly who is a direct descendant of that ancestor. If every single descendant has blue eyes, for example, it's pretty likely that this mysterious ancestor had blue eyes, too.
Having said that, proto-Germanic is one of the offshoots (the theoretical children, if you will) of a theoretical ancestor language called proto-Indo-European (or PIE, for short). Other PIE offshoots are the Italic languages (such as present-day French, Italian, and Spanish), the Hellenic languages (such as ancient and modern Greek), the Celtic, and many, many more. You can look at any "family tree" or "Stammbaum" of PIE languages to see them all. You won't see Chinese there, but you will see a suprising range of languages (such as both Latin and Sanskrit). The Germanic languages (all having the same proto-Germanic origin) include English, German, Dutch, Afrikaans, and some other languages, including more than a few that are now extinct.
The PIE people probably (judging by the vocabulary that they were likely to have possessed, given the vocabulary of all of their descendants' languages) lived inland, in a cold area, possible somewhere in what is now western Russia. For some reason, huge waves of these PIE people started migrating from their homeland; a few such waves headed east (as far as India) or south, but most headed westward toward what is now called Europe. That range of migration gives us the name Indo-European.
The proto-Germanic group is probably the last of such waves to leave the PIE homeland; this group of people may have encountered wholly new groups of people on their travels. At least that's one theory for explaining why the Germanic languages are somewhat different from the other PIE language groups. Two of the main differences between the Germanic languages and the other PIE languages are that the German languages have:
1. New words that aren't found in the other languages with the same PIE ancestor (e.g. bone, blood, soul)
2. Fixed stress on the first syllable of main words
There's a whole lot more to say, of course. Feel free to "message" me if I can help you more! The link below is just one of many internet sources that will help answer your question.