e is a number, about 2.71828. It is a mathematical constant. e also has other names, like Euler's number (because of the Swiss mathematician Leonhard Euler), or Napier's constant (because of the Scottish mathematician John Napier). It is an important number in mathematics, like π and i. It is an irrational number, which means it is impossible to write as a fraction with two integers; but some numbers, like 2.71828182845904523536, come close to the true value. The true value of e is a number that never ends. Euler himself gave the first 23 digits of e.
The number e is very important for exponential functions. For example, the exponential function applied to the number one, has a value of e.