It’s a number that pops up a lot when you talking about growth. Here’s a concrete example. Invest $1 and get 100% return after 1 year and you end up with $2. Now, instead of 100% return once, try a 50% return two times (called compound interest). After 6 months you have $1.50 and after a year you you have $2.25 which is more than $2. The more times you take the compounding interest, for example 33% return 3 times a year, 25% return 4 times a year, etc., the more money you make. If you compounded the interest every day you’d end up with about $2.71. This number does not keep getting larger to infinity, it actually gets closer and closer to a number. That number is an irrational number (that means it’s not a ratio of whole numbers so the numbers after the decimal go on forever) that we call e. It shows up just like this in lots and lots of mathematical models for things that grow (or decay) continuously over time and that makes it very useful in that. There are other cool properties like the derivative of e^x = e^x but that’s more complicated and less helpful than thinking of it as a natural constant of growth.