what does e mean in a number

It depends on the context.  Sometimes it means the number e, which is approximately:

$${\mathtt{e}} = {\mathtt{2.718\: \!281\: \!828\: \!459\: \!045\: \!2}}$$

Sometimes it is used to represent 10 to some power, when used in, for example: 5e3, which would mean 5x10³

The latter use is common in some computing languages.