Part of speech tagging creates tuples of words and parts of speech. It labels words in a sentence as nouns, adjectives, verbs,etc. It can also label by tense, and more. These tags mean whatever they meant in your original training data. You are free to invent your own tags in your training data, as long as you are consistent in their usage. Training data generally takes a lot of work to create, so a pre-existing corpus is typically used. These usually use the Penn Treebank and Brown Corpus.