What does the regular expression [a-z0-9\-] mean?

It means that it matches a to z in lowercase, 0 to 9 in digits and \ - in characters.

Explanation:

As In regular expressions such as [a-z] means that it will match between a to z in lower case.

In [0-9] means it will search from 0 to 9 in digits.

so

[a-z0-9] means that it will match all from a to z in lowercase and 0 to 9 in digits.