What does NN VBD IN DT NNS RB means in NLTK?

The tags that you see are not a result of the chunks but the POS tagging that happens before chunking. It's the Penn Treebank tagset, see https://www.ling.upenn.edu/courses/Fall_2003/ling001/penn_treebank_pos.html

>>> from nltk import word_tokenize, pos_tag, ne_chunk
>>> sent = "This is a Foo Bar sentence."
# POS tag.
>>> nltk.pos_tag(word_tokenize(sent))
[('This', 'DT'), ('is', 'VBZ'), ('a', 'DT'), ('Foo', 'NNP'), ('Bar', 'NNP'), ('sentence', 'NN'), ('.', '.')]
>>> tagged_sent = nltk.pos_tag(word_tokenize(sent))
# Chunk.
>>> ne_chunk(tagged_sent)
Tree('S', [('This', 'DT'), ('is', 'VBZ'), ('a', 'DT'), Tree('ORGANIZATION', [('Foo', 'NNP'), ('Bar', 'NNP')]), ('sentence', 'NN'), ('.', '.')])

To get the chunks look for subtrees within the chunked outputs. From the above output, the Tree('ORGANIZATION', [('Foo', 'NNP'), ('Bar', 'NNP')]) indicates the chunk.

This tutorial site is pretty helpful to explain the chunking process in NLTK: http://www.eecis.udel.edu/~trnka/CISC889-11S/lectures/dongqing-chunking.pdf.

For official documentation, see http://www.nltk.org/howto/chunk.html

Even though the above links have all kinds. But hope this is still helpful for someone, added a few that are missed on other links.

CC: Coordinating conjunction

CD: Cardinal number

DT: Determiner

EX: Existential there

FW: Foreign word

IN: Preposition or subordinating conjunction

JJ: Adjective

VP: Verb Phrase

JJR: Adjective, comparative

JJS: Adjective, superlative

LS: List item marker

MD: Modal

NN: Noun, singular or mass

NNS: Noun, plural

PP: Preposition Phrase

NNP: Proper noun, singular Phrase

NNPS: Proper noun, plural

PDT: Pre determiner

POS: Possessive ending

PRP: Personal pronoun Phrase

PRP: Possessive pronoun Phrase

RB: Adverb

RBR: Adverb, comparative

RBS: Adverb, superlative

RP: Particle

S: Simple declarative clause

SBAR: Clause introduced by a (possibly empty) subordinating conjunction

SBARQ: Direct question introduced by a wh-word or a wh-phrase.

SINV: Inverted declarative sentence, i.e. one in which the subject follows the tensed verb or modal.

SQ: Inverted yes/no question, or main clause of a wh-question, following the wh-phrase in SBARQ.

SYM: Symbol

VBD: Verb, past tense

VBG: Verb, gerund or present participle

VBN: Verb, past participle

VBP: Verb, non-3rd person singular present

VBZ: Verb, 3rd person singular present

WDT: Wh-determiner

WP: Wh-pronoun

WP: Possessive wh-pronoun

WRB: Wh-adverb