The preview feature means that users can hover over a link to explore related topics without leaving the article they are currently reading. The preview will display the first few sentences of the article, allowing users to determine the relevance of the article. They can then decide if they wish to click through.
Wikimedia says that testing shows that the numbers of people disabling the feature are ‘negligibly low’. It will study the long-term effects on reading behaviour.
“We hope this change will lead to a Wikipedia that is easier to explore, and perhaps even easier to learn from.”
Source: Wikimedia.