Power Fx is the low-code language that will be used across Microsoft Power Platform. It's a general-purpose, strong-typed, declarative, and functional programming language.
This table that shows the Power Fx function types side by side is really useful in understanding what you actually do in the apps you may have already built. No, I still couldn't tell by heart what functions are declarative vs. imperative. That won't stop me from building nice Lowcode apps, using what I know from Excel + a little extra Power.
Microsoft Power Fx overview
Power Fx: the Programming Language for Low Code and what it means for Developers