This obviously depends on the project and the company doing it - everybody has a different notion of 'free of constraints'.
Typical characteristics can be:
- no prior project that one has to build on
- no 'legacy' code or data to deal with
- free choice of language, framework, infrastructure
- free choice of programming or management techniques
- no requirements from outside the team
- complete freedom to determine scope with stakeholders
- freedom to pick whoever you want in your team
You get the idea.
Every aspect one might feel constrained by could be removed - but it doesn't have to, necessarily. Greenfield projects are designed to provide a lot of freedom, but you best ask the company offering it, what you are and aren't free to do in such a project.
- New tools that have not been necessary before
- Software related to new areas of business a company wants to expand into
- Research projects