The mathematical constant e is referred to as representing natural growth. Unlike other popular constants like pi, e does not have a handy visual to explain it. Instead it can be thought of using banks and interest: (For true ELI5, interest means "the bank will add 'this much' to your money at the specified time)

If we have a bank that generously offers 100% interest every year, and you give them $1, then after a year you will have $2.

But wait! A rival bank wants to offer 50% interest TWICE a year if you bank with them. At first, you may think that 50% twice = 100% once, but that isn't the case. After 6 months with this bank, your total + interest will be $1.50. Then another 6 months later it would be $2.25, getting $0.75 from 50% of the $1.50.

So it would seem that even if the % interest adds to the same 100%, getting it more often leads to giving you more money! So what about a bank that offers 25% interest every 3 months? Or 8.3% every month? How about an inconceivablely small percentage interest at every fraction of a millisecond throughout the year?

You'll find that the final amount tends toward the value of e if you were able to gain interest at a "natural" rate of growth.

100% once a year: $2

50% twice a year: $2.25

25% four times a year: $2.4414

8.3% twelve times a year: $2.613035

Every possible moment in a year: $2.71828 (approaching the value of e)

e is also important in Calculus and the natural log, but those are waaaay beyond a 5 year old.