e, also called Euler's number, is a mathematical constant that is used in calculus and natural logarithms. It is approximately equal to 2.72. You can probably learn more about it with a quick Google search if you search 'Euler's number'
"e" in the top left hand corner is also used to mean "exponent", when entering large numbers such as: 1.25 x 10^25 =1.25e25, which is computer language for "scientific notation".
