Hella (and its back-formation polite variant, hecka), like many new words (often called slang), displays more innovation than it gets credit for. Often, fans of prescriptive grammar and others without deep understanding of languages believe that because a word is new, used by young speakers, and/or is something that they don't understand because it is not used by their speech community, that it is meaningless and/or a "filler word" (and, in many cases, evidences language degradation). However, the word hella has unique meaning and in this way expresses a subtlety of usage that generally goes unnoticed.

The common interpretation of the word hella, both by users and non-users of the word, is as an intensifier, a class of words including very, extremely, really, and the like. Because of this belief, hella is often compared to wicked, a slang intensifier used in Massachusetts.

Hella can certainly be used as an intensifier, such as in the utterances below:

1) That movie was hella stupid.
2) She was hella checkin' you out.

However, hella can also be used as a quantifier, a class of words including some, many, less, etc. In this sense, hella roughly glosses as "a lot", though it is worth noting that it can quantify both mass and count nouns, which is unusual. Examples 3-4 illustrate this type of use.

3) We bought hella food for the party.
4) There were hella people at that party.

Furthermore, hella can be used to affirm truth value, like the word really (above I mention that really is an intensifier, but in this case, and many others, the word functions in different ways in different contexts: in the utterance "She's really smart," really acts as an intensifier, whereas in the utterance, "She really is smart," it functions to reaffirm the truth value of, "She is smart.") Example 5 illustrates the truth value function of hella.

5) She's hella from France.

It's clear that this is a truth value marker rather than an intensifier, because the proposition "she's from France" is not something that be graded the way that "she is smart" can be; it can only be intensified by having its truth affirmed.

Waksler (2000) looked at the word hella in various environments (though she did not include the truth value function) and provided the following unified analysis:

"I hypothesize that HELLA indicates a large proportion of the set denoted by the constituent it modifies. This analysis yields an interpretation as an intensifier or quantifier, depending on the denotation of the set it modifies. Syntactically, HELLA is a specifier of the Intensifier category, a more generalized intensifier than previously documented in English."

It's not clear that this analysis works with the truth value examples, however, as "from France" is not a set of which a portion can be indicated. This analysis would require further work in order to unify all uses of hella.


References:
Waskler, Rachelle. "A HELLA New Specifier" (2000) Festschrift for Jorge Hankamer, S. Chung, (ed.), http://ling.ucsc.edu/Jorge.