These are mostly the tags of the Penn Treebank tagset as used by LanguageTool,
with examples. See "new tag" for tags introduced by LanguageTool.
For more details, also see https://www.ling.upenn.edu/courses/Fall_2003/ling001/penn_treebank_pos.html

CC    Coordinating conjunction: and, or, either, if, as, since, once, neither, less
CD    Cardinal number: one, two, twenty-four
DT    Determiner: a, an, all, many, much, any, some, this
EX    Existential there: there (no other words)
FW    Foreign word: infinitum, ipso
IN    Preposition/subordinate conjunction: except, inside, across, on, through, beyond, with, without
JJ    Adjective: beautiful, large, inspectable
JJR   Adjective, comparative: larger, quicker
JJS   Adjective, superlative: largest, quickest
LS    List item marker: not used by LanguageTool
MD    Modal: should, can, need, must, will, would
NN    Noun, singular count noun: bicycle, earthquake, zipper
NNS   Noun, plural: bicycles, earthquakes, zippers
NN:U  Nouns that are always uncountable		#new tag - deviation from Penn, examples: admiration, Afrikaans
NN:UN Nouns that might be used in the plural form and with an indefinite article, depending on their meaning	#new tag - deviation from Penn, examples: establishment, wax, afternoon
NNP   Proper noun, singular: Denver, DORAN, Alexandra
NNPS  Proper noun, plural: Buddhists, Englishmen
ORD   Ordinal number: first, second, twenty-third, hundredth #New tag (experimental) since LT 4.9. Specified in disambiguation.xml. Examples: first, second, third, twenty-fourth, seventy-sixth
PCT   Punctuation mark: (`.,;:…!?`) #new tag - deviation from Penn
PDT   Predeterminer: all, sure, such, this, many, half, both, quite
POS   Possessive ending: s (as in: Peter's)
PRP   Personal pronoun: everyone, I, he, it, myself
PRP$  Possessive pronoun: its, our, their, mine, my, her, his, your
RB    Adverb and negation: easily, sunnily, suddenly, specifically, not
RBR   Adverb, comparative: better, faster, quicker
RBS   Adverb, superlative: best, fastest, quickest
RB_SENT  Adverbial phrase including a comma that starts a sentence. #New tag (experimental) since LT 4.8. Specified in disambiguation.xml. Examples: However, Whenever possible, First of all, On the other hand,
RP    Particle: in, into, at, off, over, by, for, under
SYM   Symbol: rarely used by LanguageTool (e.g. for 'DD/MM/YYYY')
TO    to: to (no other words)
UH    Interjection: aargh, ahem, attention, congrats, help
VB    Verb, base form: eat, jump, believe, be, have
VBD   Verb, past tense: ate, jumped, believed
VBG   Verb, gerund/present participle: eating, jumping, believing
VBN   Verb, past participle: eaten, jumped, believed
VBP   Verb, non-3rd ps. sing. present: eat, jump, believe, am (as in 'I am'), are
VBZ   Verb, 3rd ps. sing. present: eats, jumps, believes, is, has
WDT   wh-determiner: that, whatever, what, whichever, which (no other words)
WP    wh-pronoun: that, whatever, what, whatsoever, whomsoever, whosoever, who, whom, whoever, whomever, which (no other words)
WP$   Possessive wh-pronoun: whose (no other words)
WRB   wh-adverb: however, how, wherever, where, when, why
``    Left open double quote
,     Comma
''    Right close double quote
.     Sentence-final punctuation (in LanguageTool, use SENT_END instead)
:     Colon, semi-colon
$     Dollar sign
#     Pound sign