Consider this analogy: take 8 drinking glasses and line them up on your table. Because we are optimists, we will say that any glass that is more than 1/2 full is a ONE,and any glass that is less than 1/2 full is a ZERO. Because each glass has 2 'states' (1 and 0) and because there are 8 of them, I can use this to store 256 different combinations (in computer terms this is called a 'byte').
To read each bit, I stick my finger into the glass, and see if its half-full. Every time I do this, a little water sticks to my finger, so after many many 'reads' the value stored in the water can change. In Flash device, how ofter you can stick you finger in the glass is called the 'Endurance' of the flash (typically 1 million times) Note that the water in my glasses can also evaporate, so that after a long time my glasses are all empty. This is equivalent to a Flash property called 'Data Retention' -which can be 10s of years for typical flash devices.
This is pretty simplified, but I think you can get the idea.