So consider the starting time of a project. The features that are required for the project to succeed have been generated (including the trade off about the ones that are critical for the project vs. the ones that can be cut when there are schedule risks), and there is a need to ensure that there is a formal kick-off for the project. What do you do in a formal kick-off ? Well, one of the key items is to ensure that the team knows what the project is about, what are the key features that will be there in the project, and other such details. Typically the product manager will give a presentation to the team about the aim of the product, the market, the revenue earned by previous versions of the product and the target for the current version, and a high level description of the features that are sought to be included in the project. For many of these features, the further detail that the product manager can give is about how these features were derives, whether these be from competitor analysis or specific requests by customers, or generated based on ideas by the team.
However, as part of the initial kick-off, there are many other discussions that need to take place. For making the software development project successful, there is a lot of support that is needed from other teams. There are teams that prepare the build and the installer, there are teams that work on the documentation and help files, there are teams that deliver components that are required for specific functionality in the product, there are teams that work to translate the product in other languages. Most modern organizations have these kind of dependencies; it is no longer possible to have a situation where a single team does everything that is required for the team (for example, if you have a product that uses video formats, you would rather be using a common component that uses different video codecs to work with these formats rather than write code for working directly with these video formats (it may not even be possible to write code for all these video formats due to technical and commercial limitations)).
It is easier to ensure that you have the support of your core team and more difficult to ensure that all these supporting teams are providing the required support. For this purpose, before the kick-off, you would need to work with all these teams, providing them details about your requirements and the schedule in order to get a commitment from them (in some cases, where there is a clash due to priorities for support, you may need to escalate before you can get the desired commitment). In all cases, unless you can get the required level of support from these teams in terms of commitment, your project is already at a certain amount of risk that you will need to manage in one way or the other.