I’m sure you’re intrepid enough to cross an empty room blindfolded? How about a busy freeway? Well the difference is one of probability. If you’re going across an empty room blindfolded, an enormous python could quietly sneak in and without difficulty strangulate, and eat you, preferably in that order. This, most sane people would judge to be highly unlikely, unless this room was adjacent to a snake-house. Therefore this would hardly be considered a dangerous endeavor, quite unlike crossing a busy freeway blindfolded, an activity which could easily impact ones dinner plans. The moral is that one is implicitly aware of probabilities, because we have a pretty good intuitive understanding of risks.
But how good are we at quantifying probability? Well first off, what does it mean? Suppose you cross a room blindfolded 1 billion times. 19 of those times you trip over your shoelaces and break your nose. 42 of those times you decide to skip across the room and stub your toe. And 12 times you are eaten by a python. That’s 19+42+12 = 73 times that there was some mishap. The rest of the time you’re fine. The probability of crossing safely is then roughly (see below) (billion-73)/billion = 0.999999927. Now you decide to cross the freeway blindfold. You’re not as patient and only try 100 times. You end up only managing to get across unscathed 32 times. Therefore the probability of getting across safely is roughly .32 .
This in a nutshell is what probability is.