Item in SuperMemo is a piece of knowledge that you want to remember. It usually has a form of a question and the answer. In incremental reading, items are usually made with the help of cloze deletion.
The main difference between an item and a topic is that an item actively tests your memory (e.g. with a question), while a topic is used for passive review only (e.g. for reading, viewing, watching, etc.).
See also: Glossary