Joel Spolsky's classic: The Law of Leaky Abstractions is still well worth a read today.
Many of today's high-level tools abstract away the low-level details to such an extent that 99% of the time, you don't even need to know they're there. But it's that 1% of the time that's the doozy. When everything suddenly blows up because you accidentally crossed an invisible line you didn't know was there... how can you figure out what went wrong and how to fix it?
The low-level detail leaks through.
So often, even though you're using an abstracted, high-level tool, you often still have to learn all the lower-level stuff it was helpfully abstracting way.
An example would be a need to learn sql in order to understand joins and includes (and how they go wrong) in Rails' Active Relation