Before you can understand diction, you need to understand the difference between denotation and connotation.

Denotation - the dictionary definition of a word

The exact definition.
No feelings/emotions/experiences.

Connotation - the implied meaning of a word

The feelings/experiences associated with a word.
Sometimes different for each person.

Note on using connotation:

Connotation is used to imply the subtle differences between words that are difficult to recognize sometimes. For example, even though "childlike" and "young" have a similar denotation, "childlike" implies immature behavior, whereas "young" seems to refer simply to age.

Example of denotation vs. connotation: "Home"

To help illustrate the difference between denotation and connotation, let's take the word "home" for example.

Denotation: Home (noun) - one's place of residence

From Merriam-Webster Dictionary
Note: this is a very formal way of describing home

Connotation: Home - warm, associated with friends/family, safe, familiar

These words are not part of the dictionary definition, but are things often associated with "home." The associations shape your understanding and often use of the word.
Other people who have a tumultuous home life might describe "home" as unfriendly, unsafe, cold, or dark. That is why connotations can vary and should be used carefully.

Still, a little bit confused? More information:

Article (& quiz) on connotation: https://literaryterms.net/connotation/
Article (& quiz) on denotation: https://literaryterms.net/denotation/
See the next pages for more resources