Packaging is the step prior to sharing or releasing a tweak/application/tool/library/theme. Here you put all your project files into a single nicely wrapped file that others can install on their devices.
Tools
To do: add existing tools, on which platforms they are available, how to use them.
DEBIAN folder
Control file
To do: add layout of fields, what do they mean, how they are used. Refer to saurik's post and links at the bottom.
If you need to list dependencies or conflicting packages, Debian's packaging manual may be useful (because Cydia packaging is based on Debian packaging): Syntax of relationship fields, Dependencies, Conflicts -- or if you're submitting this package to a default repository, you can just ask your repository maintainer for help with this.
Other files
To do: add information about preinst, postinst, extrainst_, prerm, postrm files. Consider paste for base explanation/example. Explain how it fits into Debian packaging (how it interacts with apt/aptitude/dpkg/Cydia). In the Debian packaging manual: basics and more info.