What is the meaning of campaign?

The origin of this term is military, and comes from the French Word for a field. Previously, an army could only leave its barracks and go out into the field when the weather and road conditions were favourable. When favourable, the army would leave the barracks, march out into the field, fight a number of battles, then return to its barracks when the weather became unseasonable. A "fighting season" of this kind was called a campaign, usually with a specific objective which could be reached within the season, such as the capture of a particular place or the trapping of another army.
The non-military senses of the Word derive from this. A political campaign is a series of actions directed to the limited and time-specific objective of winning a particular election. A sales campaign is a series of actions designed to promote and sell a product or service within a limited time.