People are downvoting you because quirky/jokey super-colloquial language like “wossis mean? TIA” is hard to understand, and also just doesn’t really mesh with the vibe of the site.
Value space is the set of values a type can have. A boolean has only two values in its value space. An unsigned byte has 256 possible values, so does a signed byte.
A string enumeration has a limited number of values. E.g. type A ("Yes" | "No" | "Maybe") has three values and is a superset of type B ("Yes" | "No"). A function that accepts type A can also accept type B as valid input.
If the value space is defined by a regular expression, as is often the case, the mentioned library could be used to check, at compile-time, which type are subsets of others.
"For example, e-mails and urls are a special syntax. Their value space..." seemed to talk about the 'value space' of strings (these being e-mails and urls), not types (of e-mails and urls), which confused me.
It is bout the 'value space' of strings. Think of all possible strings. That is the entire value space of strings. Not every possible string is an email. Only a subset of this value space is a valid email. This subset is the 'value space' of strings which are valid emails.