"Rapid boil" means to heat a liquid until it starts to bubble and boil quickly, often indicating that it has reached its highest temperature. Full definition
Reduction Sauces are produced by rapid boiling of gelatinous stock to produce a thick, clear sauce. (westonaprice.org)
I find the more rapid boil and longer I cook it, the chewier the Carmel! (naturalsweetrecipes.com)
Add the wontons to the pot (no more than a dozen or so at a time) and lower the heat so that it's at a steady but not rapid boil. (theculinarychronicles.com)