This is a very generic simplified explanation. In it's essence it is a numerical system based on 2 (0,1), unlike the numerical system we are accustomed to which is based on 10 (0,1,2,3,4,5,6,7,8,9). However the best way to think about it is on and off. On is 1. 0 is off. Digital circuitry only understands 2 states on and off. So binary code is how you talk to ICs on their level. All computer programs, usually through an interpreter are converted to binary code otherwise your processor, memory etc.. would not understand what was given to it or what to do with it.