This is where you convince people to sign your petition. Use it to describe the problem, identify the solution and who can make it happen, and explain how taking your action helps spur that solution. You also want to provide essential facts to support your argument. Many unfamiliar but interested people won't read a whole essay, so be sure to give context as succinctly as possible.
Crisis: Start by going straight to the thing that needs changing without beating around the bush. It may be helpful to articulate it as a crisis that demands our response.
Our role in change: This is the story of how the petition leads to the solution being enacted -- think about who you're targeting and what role public pressure can play in their decision.
Ask: Make sure to invite the reader to take your action. It’s also helpful to describe any further specifics here of ask.
Supporting information: Is there anything supporters need to know? Perhaps there's a history here you need to explain or a common criticism to put to rest.
Resist the temptation to give a long introduction instead of cutting straight to the problem (it's hard!).
Explain the time-element -- why now? What's the urgency?
When in doubt, cut -- if you can't tell whether something's essential to include, it probably isn't.
Keep your audiences in mind -- you need to motivate people to sign, while also keeping in mind that your target may visit the page (especially if you're creating a lot of pressure).