Lemma refers to the abstract lexical category that is formed by a lexeme base plus its inflected forms, e.g. the lemma GO includes go, goes, going, went, gone. Lemmas for nouns contain the singular and plural forms, for adjectives the positive, comparative and superlative forms, and pronominal lemmas group the different cases of a pronoun (e.g. you, your, yours).
Thus, lemma searches shows all realisational variants of a word. To use them, put a word in square brackets or, using the new BYU syntax, in all caps. For example, [tree] will return tree and trees, [nice] gives you nice, nicer, and nicest, and [swim] will return swim, swims, swimming, and swam.
According to their definition, lemmas share the same part of speech. However, the search function in the BYU corpora is not limited to the part of speech: [house] or HOUSE gives you house, houses, and housing (which can be either verbs or nouns) along with housed. Therefore, if you want to only find a certain part of speech but your search term is ambiguous, add a PoS tag.
The process during which a lemma is assigned to each word is referred to as lemmatisation.