What does this mean: Topic is Locked?

A topic that has been locked is identified by this image:

The topic can still be read but no more replies are accepted. Why should this be required?

1. Some topics can extend for several pages and become too large to follow easily.
2. The point of the topic has been reached and resolved and further discussion becomes irrelevant or confusing.
3. The moderator decides to close the topic due to poor conduct or sensitive disclosures.

This Q&A was found on XOOPS Web Application System : https://xoops.org/modules/smartfaq/faq.php?faqid=55