Preferences are the seventy or so user options for browsing, editing, searching, notifications, and more. A link to your Preferences page is available at the top of every Wikipedia page when you have an account, alongside links to your other account services such as your Contributions. Another way to get there is by navigating to the Special:Preferences page.

This page is a selective overview of user preferences. For full details see Meta:Help:Preferences.
This is Help:Preferences. The talk page for Special:Preferences is at Wikipedia talk:Special:Preferences.

Once at your Preferences page, you can control much of the Wikipedia user interface through the many feature settings provided by MediaWiki (the software of Wikipedia): skins, plug-ins, date formats, a signature, and more. For example, you can select to be prompted to enter an edit summary if you forget to. The Preferences page also presents a link to customize your CSS to adjust your page style details.

Plus, there are hundreds of user tools to explore, many listed at Wikipedia:Tools § Browsing and editing, that can override or tweak preferences provided by MediaWiki. Each account has its own Custom JavaScript page where the tools install themselves, and where you can help develop them.

At their site, MediaWiki maintains a browser compatibility matrix. The time you spend in getting an account, setting your preferences, finding tools and testing your browser (before you ever lose an edit) will pay off.

Feature requests can be made, and bugs reported, as described at Wikipedia:Phabricator. The current set of preferences are largely the result of issues raised by newcomers at places such as the Village pump, and driven by regulars who hold long discussions and collectively drive issues.