While reading modern JavaScript code, you might have come across such things as Array.prototype.map(), Array.prototype.filter(), pure functions, higher order functions, the spread operator, etc. Basically, all the things that React and Redux made us write willingly or unwillingly. I am not going to spend the next 2-3 hrs going into finer details about FP. Instead, I am going to link you to 2 great articles I found on this topic.
Functional Programming (or FP) is a programming paradigm, a collection of guiding principles to write code in a particular style. Any developer who strongly adheres to this way of writing code will tell you that it eventually leads to code that is more readable, maintainable, testable and bug-free. In saying that, they would probably be right. Creating in React and GraphQL has made me a big fan of the declarative programming style. FP promotes declarative over imperative. Again, don’t worry too much about all the jargon. Once you’ve read the following articles, you’ll have a good understanding yourself.
FP is just one of multiple paradigm that JavaScript supports (other ones being event-driven, object-oriented and prototype-based; last two being forms of imperative paradigm). There are several other languages that support FP, most notably Haskell which supports ONLY the functional style. The currying technique is an essential ingredient in creating functional code, and is supported in a lot of modern languages including JavaScript.
Interestingly, there was a (famous?) guy named Haskell Curry who is the namesake of the language and the technique described above. Haskell. Currying. Get it?
Enough said!
Here are two excellent articles that introduce functional programming superbly. Read the first one first, after which all code examples in the second article will be immediately understood (really, Avi Aryan has done a great job with those examples).
Master the JavaScript Interview: What is Functional Programming?
Intro to Functional Programming: JavaScript Paradigms | Toptal