Functional programming boils down to 2 important ideas.

    1) Referential Transparency - Basically, you can take an expression in your program, and replace it with the result of that expression. and your program will run 
    the same, every time.  

    2) Immutability - the idea that data cannot be deleted or changed.

Having a language/runtime that truly embodies these two principles completely eliminates entire classes of problems in regards to parallelism and concurrency.

Some languages and their implementations are very ergonomic to programming in a FP style, some are not. ( Im looking at every imperative lang that has lambda semantics and now says they are FP. )

I think people get hemmed up talking about what FP is. There's nothing in there, though about Category Theory, Algebraic Data Types, Static or Dynamic Typing Disciplines, etc.