Commas are required between items in lists (of three or more things). You need commas between verbs, nouns, pronouns, adjectives, adverbs, prepositions, and conjunctions, whenever three or more occur in a list, e.g.
I like to sing, dance, and play piano.
The dog was small, fluffy, and had muddy paws.
Mary, Sally, and Gloria were going to the beach that day.
Additional information:
There are three main conditions for using commas: to separate items in a list; after signal verbs and before dialogue/quotations; and to ‘set apart’ parenthetical elements, including to set apart subordinate clauses that begin sentences, after introductory elements, and before tag questions.