Grimm's Law is a linguistic rule which tells us about consonant shifts that occur somewhere between Indo-European and medieval Germanic languages. I'm not going to give a more complicated explanation than that because I would rather just give some examples which demonstrate one of those consonant shifts from words beginning with a voiceless stop in Proto-Indo-European and Mediterranean languages (in this case P) to a corresponding aspirant (in this case F).
fire in Greek is πυρ (pur), it's where we get our word pyromaniac
father in both Greek and Latin is pater (πατρης), it's where we get our words patriarchy and patricide
foot in Greek is πους (pous) and in Latin is pes, it's where we get our words pedometer and podiatrist
fowl is likely related to the Latin word pullus which means rooster, a word we acquired again in our language through the Norman invasion in the form of poultry
fell is a Germanic word for an animal skin and where we get our word felt from—in Latin, animal skin is pellis
fish in Latin is pisces, which is where a lot of people get stupid ideas about reading the future from right now
How cool is it that we have an idea of what all of these words may have been in Indo-European?