Front End Loading (FEL) is a term used to describe the phases of a project that precede execution.  In the context of a stage gate process, it usually covers the Appraise (or Identify), Select, and Define stages.

Its purpose is to instill the discipline of doing rigourous analysis and evaluating alternatives/options before the execution begins (when the cost of mistakes are much lower) rather than changing your mind though the execution phases (when the consequences are much higher).  In other words, it is a methodology to ensure that you do your analysis thoroughly before you make decisions, leading to better quality decisions and higher value outcomes. 

Final investment decision (FID) typically marks the culmination of the FEL process.