How is the E(X) of a Poisson distribution lambda? I was recently learning about the Poisson distribution, and was very perplexed about the E(X) equaling lambda. Like how did lambda even come into the picture here, isn't it a symbol of wavelength? 
Also isn't wavelength just a smaller unit of distance? In that case isn't saying E(X)=5 cars per hour(assume) wrong? Because Distance=Speed * Time, and saying per hour signifies division.

Thanks in advance for solving my doubts. If possible please explain everything in layman's terms.
The probability mass function for a Poisson-distributed random variable $X$ is $f_X(x)=\dfrac{\lambda^xe^{-\lambda}}{x!}$. The expected value is a sum over all possible values of $x$, the natural numbers (non-negative integers).

$$\mathbb{E}[X]=\sum_{x=0}^{\infty} x f_X(x)$$

When you evaluate that sum, the result is $\lambda$.

Yes, $\lambda$ often means a wavelength in physics. That’s just notation, however, and easily could be $\xi$ or $\beta$ or $\gamma$. Your physics knowledge will also tell you that $\theta$ is an angle, yet statisticians often use $\theta$ as a parameter. For the Poisson distribution, $\lambda$ denotes a rate, and this is how you get five cars per hour as the average rate of cars passing through a remote intersection (or whatever they’re doing).

EDIT

I like JBStatistics. He derives the expected value of a Poisson distribution, if you’re interested in seeing how that series calculation works: https://youtube.com/watch?v=65n_v92JZeE.