1. Reactors that are water-cooled (or water-moderated) have a parameter called a void coefficient. As the water flows into the reactor, some of it may boil before leaving, producing bubbles or a void. If the core is producing too much heat, this can become significant. If the formation of these voids causes the core to slow down, the void coefficient is negative. Generally, (in the western world) a slightly negative value is desirable. Too large of a magnitude means the control systems have to react very quickly.
    
   The RBMK design had a void coefficient that was positive and very large. This meant that if the core was running hot and voids started to form, the reaction rate would (assuming no intervention) increase at a rapid pace. That would make the core hotter, boil more water, form more voids, and so on.
    
2. fuck it, i cannae be arsed. Look up how the control rods were designed, if you so care.
    
   TL;DR: RBMK (as implemented at Chernobyl No. 4), under certain conditions, ran metaphorically balanced on a knife edge of safety. The operators sucked, but the reactor design made it far worse.
    
   _{Condescending rant over, you may now leave your seats.}

@bahnpirat
if you’ll note the “РБМК” there? Yeah, that’s “RBMK” in English.