It means 'in essence' or that what we call 'intelligence' is essentially just curiosity. If you boil something down, you remove all the extra stuff leaving behind the critical elements.

"is nothing more than" "is basically" "is essentially" "is fundamentally"

There are many ways to say this: "a lot of what people call intelligence is really just curiosity" "a lot of what people call intelligence is actually nothing but curiosity" "a lot of what people call intelligence is in reality just curiosity" "a lot of what people call intelligence is in fact really (or 'just' or 'actually') curiosity"

Another way to say it would be "what people call intelligence is just curiosity at the core". The saying imagines you could actually put intelligence in a pot and boil away all the water. What would be left is curiosity.