Hard Boiling is a hypothesis used in the RNA Lab by some players while proposing puzzle solutions. To be more precise, hard-boiling an RNA is the process of 'sharpening' the RNA's melt plot so that the RNA denatures from its shape very quickly over a few degrees instead of gradually over a long series of temperature rises. The idea comes from an observation that Quasispecies made while reviewing past lab winners and noted that many successful labs had very sharp melt curves associated with them.
As it stands now, the hypothesis of hard-boiling is yet unproven, but the general idea of how it works is that the RNA fold denatures back into a strand very abruptly and very quickly, which minimizes the number of acceptable folds the RNA can take on. It is possible that the reverse is true - that because the number of different folds the RNA can take on is minimized, it denatures quickly. Either way, the end result is that there are only a few acceptable folds that the RNA will take on at any given point in time, reducing the possibility of misfolds and mispairings while also increasing the stability of a given fold.