what is e?

"e" is NOT an "imaginary" number....  is the  base for the  natural logarithm  (just as 10 is the base  for the  "normal" log)

It equals  ≈ 2.71828.....   (it is irrational so it has a non-repeating, non-terminating decimal part )

This number  comes in very handy for many scientific and financial applications  (such as growth/decay functions)

The concept of $e$ comes from compound interest. Suppose a very generous bank offers you a $100\%$ interest rate, which would give you twice as much as what you put in every year. If you compound the interest quarterly, you would recieve $25\%$ of your $\bf{current}$ amount every three months, eventually gaining around $(1.25)^4\approx 2.44$ times of what you invested. 

So suppose that you compounded your money even further, recieving interest every day, hour or even second. Would you achieve infinte wealth? Sadly, no. $e$ is defined as the limit of this growth, which is 
$$\lim_{n\to \infty}\left(1+\frac1n\right)^n = 2.71828182846\dots$$

Despite its apparent disconnection from other areas of mathematics, $e$ turns up quite a lot in calculus and complex numbers. For example, you may have heard of the famous equation $e^{i\pi}=-1$. 

Some other facts about $e$:

• It is both irrational and transcendental, that is, it cannot be expressed as $\frac mn$ where $m$ and $n$ are relatively prime integers. Nor is it the root of any polynomial with rational coefficients. 
• The growth of the function $e^x$ is exactly itself, that is, the slope of the tangent line at any point $\left(x,e^x\right)$ is exactly equal to $e^x$, or in other words $\frac d{dx}\left(e^x\right)=e^x$. You will learn more about it when you get to calculus.