"Winter rules" is not codified in any of the 34 rules that make up the Rules of Golf; it exists only in an appendix to the rules, where there is a sample local rule for use by courses, clubs or committees.
Generally, winter rules are put into place during, of course, the winter, when adverse weather conditions can impact the golf course itself. Or winter rules - a k a, preferred lies - might be put into place at any time of year on a course that is suffering damage; for example, if extreme drought kills large swathes of fairway.
Here's how it works: You show up a golf course and are told (or read via signage) that "winter rules are in effect today." You tee off No. 1 and hit the fairway. But when you reach your ball, you discover it sits on bare hardpan despite being in the fairway. Under the most common traditions for preferred lies, you have the right to move that ball up to six inches in any direction to improve the lie, without penalty (picking it up and placing it).
Important: You can only apply "winter rules" if the course or competition has invoked the local rule. You cannot simply take it upon yourself to start improving your lies because the fairways are thin. (That's called cheating.)
If winter rules are in effect, the course or committee should spell out exactly what that means and inform all golfers. If you carry a handicap, rounds played under winter rules must be reported for handicap purposes. But if winter rules are in effect, you are not required to abide by them; you can choose to play it as it lies.
To learn more about winter rules, read our article in the Golf Rules FAQ: "What are winter rules (preferred lies)?"