Revenue is money earned by a business, or income received by the government from taxes. The government is always interested in dreaming up new sources of revenue, and so is the average head of a company unless she doesn't want to be the head anymore.
Revenue is from a French word meaning "to return or come back," and dollar returns are always a welcome thing. Declining advertising revenue will doom your website, no matter how great it is. If your lemonade stand is not making a profit, you might consider lowering costs. Maybe you could get cheaper lemonade mix? Or you might look for more creative and exciting ways to increase revenue––like raiding the pantry for cookies you can sell as well.