Lemma is the head word of a lexical family, that is, the form of the word that is defined in a dictionary.
A lemmatizer is a program that identifies the lemma or lemmas of a word.
Most of the words we use are inflected, which means they vary in gender (female or male), number (singular or plural), person, tense, and mood. The lemma, therefore, is the form that conventionally represents all other inflected variants of a word.
This tool identifies all possible lemmas for a single form. Additionally, it allows for the identification of the lemma of an inflected word, the part of speech, and the meaning of the inflection.
For example, for the word gatitos, the lemmatizer identifies its lemma, gato, and that it is a masculine noun, plural number and is a diminutive form. Similarly, when entering the word leerá, it indicates that the lemma is the verb leer, in the third person singular, future tense of the indicative mood.