A loose sentence is commonly defined as a sentence starting with a predicate statement, with additional information based on that statement. That's a pretty loose definition, and doesn't include the fact that the subsequent information may include additional qualifiers and information. Loose sentences are by definition complex, and usually long. In modern theory, loose sentences aren't good usage, but were common among great writers of the past. These sentences are often structural in story lines, and can contain a lot of information.
Examples of Loose Sentence:
It was a hot day, and George decided to leave work and go to the beach, where he knew his friends would be arriving later.|
The wildcat looked briefly at the two humans, seemed to sneer with a raised lip, and stalked off back into the supermarket.