Taken literally, "einfallen" means "to fall into". But the meaning that makes it one of the most useful German verbs is a little more abstract ;)
einfallen, der Einfall, einfallsreich, reinfallen
fallen, auffallen, ausfallen, befallen, entfallen, einfallen, abfallen, anfallen, verfallen,...
The core idea of this root was:
It doesn’t have any other relatives besides to fall and fallen (and “to fell” and “fällen”) in German and English and it is notably absent from the Latin branch of the Indo-European family. There, the word for falling was *kadere, which is the origin of words like cadence, incident or case.