PoS stands for part of speech, such as nouns, verbs, pronouns, adverbs, adjectives… You can instruct the BYU interface to only look for nouns in a corpus using _n*, for instance, or NOUN in the new search syntax. Combinations are also possible, e.g. flat_n* only finds instances of flat used as a noun instead of as an adjective.
The tags can be inserted by clicking on the menu to the right of the search bar (or by knowing the right abbreviations). However, the tagging is done automatically and thus doesn’t always have to be 100% correct. It is recommended that you always check your search results in the list display to make sure they are precise.
Due to the new search syntax, there are now two ways of specifying the part of speech: either by using the abbreviations from the drop-down menu on the right or in capital letters, e.g. NOUN. However, it seems that the abbreviations offer more specific categories, e.g. past participle forms of verbs or different types of pronouns. Beyond that, only the older tags can also be attached to search strings and do not just work in isolation.
You can find lists of tags and their explanations here: