Forth is a language which was designed in the 1970s, originally for control of the National Astronomical Observatory at Kitt Peak, west-southwest of Tucson, Arizona.
It was later also used for machine control and general computing in various industries. It was ported to the 8086 IBM PC (and compatibles) in several different configurations. It was also ported to other computers as well, including the Radio Shack 6809 Color Computer. My CF83 version for the Color Computer is presented on this site at my CF83 System Page.
Most computer languages use infix notation (aka Algebraic Notation), but Forth uses postfix notation (aka Reverse Polish Notation) instead.
Generally: Forth is harder to learn than BASIC or Python, but it's faster. On the other hand, while Forth is generally easier to learn than Assembly Language, it's not as fast as Assembly.
You can also explore more about the differences between postfix, infix, and prefix notation at the University of Manchester's site.
Probably the best source of information about Forth Programming is the website of the now defunct Forth Interest Group (fig).