He is the one responsible for the "e" you see in your calculators. It is a constant that is approximately equal to 2.71828.
Basically it means this:
The base of the natural logarithm.
From "e" he derived "Exponential Function" from f(x) = e^x. It is used to model a relationship in which a constant change in the independent variable gives the same proportional change (i.e. percentage increase or decrease) in the dependent variable.
I've found this nice math video that is got to do with youtube: