A rebellion is when you rise up and fight those in power, usually on a large scale. In "Star Wars," the good guys fighting the powerful, evil empire were known as the rebellion.
If you know that a rebel is someone who fights authority, then you won’t be surprised that a rebellion is something similar. Rebellions can be wars, particularly wars for independence from the ruling government, but many rebellions never come to armed conflict. Most of the time they're wars of words, or even just small-scale rule-breaking that is supposed to make a larger point. Students might wear shorts to school, for example, as a rebellion against the dress code.