What is "a part of speech"?
A part of speech is a gramatical category which defines a word's function in a sentence.
A noun is a word identifying an object or idea (a lake, love, a man etc.)
A verb is a word telling what is the subject doing or what is happening to it (to run, to write, to seek etc.)
An adverb is a word describing a verb (he ran quickly, we often go for a walk, etc.)