A Development Plan says from a development perspective, how you are going to develop, test, audit and deploy your smart contracts. It is a very top level document that shows you have thought through the whole development process.
The Development Plan says what you want to do before you do it. You can present this plan with the System Description to an auditor or for a Preliminary Audit. The idea is to get critical feedback before the main work begins..
Describe the team and their expertise
Define the language (Solidity/Viper/Flint etc.), libraries, tools
How will you structure the contracts?
How much on chain, how much off?
What is your test plan?
How long do you expect the development to last?
Deployment Phases (Truffle, Rinkeby, MainNet Beta, Final MainNet Release)
Audit Phases (How many audits and in when?)