Attributive
An attributive modifier is one that relies on being adjacent. (unlike e.g. predicatives, which rely on a copula (linking verb), often "be", "seem" or similar)
The word 'Attributive' often refers to attributive modifiers specifically for nouns.
Note that since these are always modifiers in a noun phrase, they can be removed from the sentence without changing its structure.
Attributive adjective
For example:
- in "the blue sky," blue is an attributive adjective
- in "the sky is blue," blue is used as a predicative adjective
In many languages, attributive adjectives usually come before the noun they detail. A language's grammar may also have adjectives that come after the noun (ignoring institutionalized phrases for a moment). In English, adjectives that modify pronouns do this; consider 'She is someone useful.' These postpositional adjectives are also called 'postpositive'.
Attributive noun (Noun adjunct)
Nouns can also be attributive. Like attributive adjectives, this often serves an adjunctive function.
In many cases, they are the first element in a compound noun, such as chicken soup, and name brand. In other cases, you can say you only make a noun phrase by modifying a noun, and do not end up with a compound nown. This distinction is a fuzzy one.
In various cases, such as 'love letter,' you can argue this to be somewhat collocative, but nonetheless, 'love' is used to indicate a type of letter.
...versus posessives
In some cases, a posessive would be practically equivalent, preferred, or (in)correct.
Consider "That's the Celtics' mascot" and "That's the Celtics mascot." The first is attributive, the second posessive (plural posessive; sometimes singular posessive is a third option), but both of these will be understood as meaning express the same thing.
There is also Mother's Day and Veterans Day. Usually these act as names, and so have one correct form, but the correct spelling is settled by convention, and convention may be confused.
This is also true for names for organizations, such as a "Bankers' Organization of Sometown," which are names so should not be corrected to your own sense.