What does Wax and Wane mean?

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This expression originally came from the stages of the moon. The moon starts as a small part on the right side and grows (waxes) until it its a complete circle (called a "full moon), then starts to get smaller on the left side (wanes) until it is completely dark (called a "new moon"). As an expression, it means something that increases and decreases in cycles, like the economy or interest in a certain fashion or the popularity of something. The phrase is not very common because the situation for it is not common, but everyone knows what it means.
"Skinny jeans seem to wax and wane in popularity. Some years everyone is wearing them, then the next year, no one is."
"How successful that politician is depends on whether the economy is waxing or waning."