John: "Darling, do you want to go out to the show tonight?"Notice how Laura didn't respond to John's question by saying, "No, I don't want to go out to the show tonight." What she actually said — her locutionary act — was "I'm feeling ill."
Laura: "I'm feeling ill."
John: "That's ok. You stay there and I'll make soup."
An illocutionary act is what a person does in saying something else. Locution is speech. In-locution (in speaking) becomes il-locution through phonetic assimilation. In saying that she feels ill, Laura was telling John that she doesn't want go out.
Beyond communicating the state of her health and the answer to John's question, Laura accomplished one more thing through saying "I'm feeling ill." She got John to make her some soup. A perlocutionary act (per-locutionary, through speaking) is focused on the response others have to a speech act.
These terms from J.L. Austin's 1962 book How to Do Things with Words are used extensively in philosophical literature today. And in fiction, having a character who is deaf to the illocutionary force of language is always good comedy.