What does Erlang's export syntax /x mean? Why the slash and then a number?

/1, /2, /3 etc are referred to as the "Arity" of the function, Arity meaning the number of arguments accepted by that function.

In Erlang, two functions of with the same name but with different arity are two different functions, and as such are each exported explicitly. To quote the Erlang documentation is says:

A function is uniquely defined by the module name, function name, and arity.

For example, if you have two functions:

do_something() -> does_something().

do_something(SomeArg) -> some_something_else(SomeArg).

And at the top of your module, you had only

-export([do_something/0]).

Then only the do_something with zero arguments would be exported (that is, accessible from other modules in the system).