Can someone explain for me?
Forth I assume uses reverse polish notation: arguments before the operator.
3 4 +
for example, would return 7.
Forth uses RPN so the "verb" is last.
That is, you provide the data first (I and Forth) and the command (heart) last.
I found https://www.explainxkcd.com/wiki/index.php/Main_Page quite useful for that.
A different way to say the same: in forth, words get pushed on the stack, and popped from the stack by words that take arguments: