A roadmap is a visual representation of how the product will evolve in the future. It usually includes:

• Planned versions.
• Estimated dates.
• Planned features.

In theory, it helps align teams and manage expectations. But it is important to use it with caution. 

It is essential to understand that a roadmap is a hypothesis that will be revised.

• The further out in time, the less detail.
• Communicate direction, don't give exact dates.
• Update it as you learn something new.

The roadmap adapts to reality, not the other way around.