If you'd like to look up more about the origins of PIE, look up the Kurgan Hypothesis, which suggests that Proto-Indoeuropean originated on the steppes.
Basically everything we know about PIE, we know from looking at its descendants. If a word appears in multiple unrelated branches, it's probably from the common ancestor. Particularly if there's consistent sound changes on one or more branches.
For example, it seems that a lot of PIE words with a p morphed into f in germanic languages. So, given the English father, Dutch Vader, Old Saxon fadar, Latin pater, Sanskrit pitar, Old Persian pita, etc. we can figure out that father goes back to some original PIE word which was probably something like pəter.
Similarly, we see similar words for salmon both in Germanic and Slavic. And in the extinct Tocharian language, the word for fish in general was laks. Lox originating only 1500 years ago means that the Slavic and Tocharian would be a pretty strange coincidence.