The denotation of a word is its dictionary definition. The connotations of a word are its emotional associations. For example, cheap and frugal both refer to a reluctance to spend money, but they have different connotations, as the following sentences show.
Her grandfather was cheap, [negative connotation] Her grandfather was frugal, [positive connotation]
Clear writing requires words with both the most accurate denotation and the most appropriate connotations. If you have trouble finding the right word, look in a thesaurus for synonyms. But be sure to check a dictionary for meanings and usage examples of any unfamiliar synonyms.