No idea, sorry. It has to do with GNU programming, but I don’t have any experience with that. I did a quick search and found the article I attached, hoping it would be helpful.
Looks like an example of a recursive function. It takes a string as an input parameter and returns the string in reverse order.
Looks like the line you have shown is actually the example in the article @bkpaladin linked, just missing the initial string.
“$ echo “Don’t Panic!” |”
So the last bit after the -| is the result of the function: “Don’t Panic!” written backwards.