The base form of a word, particularly a verb, is the simplest form it takes. The term is most commonly applied to verbs. Verbs form the word class with the most inflections. The base form is the verb with no inflection. 'Do', for example, is the base form of the verb which whose other forms are 'does', 'did', doing' and 'done'. All these are inflections. Regular verbs like 'talk' have four forms: the three inflections 'talks', 'talked' and 'talking', which are all derived from the base form talk.
See also infinitive.