A product roadmap is a high-level plan that shows how a product is likely to grow over time. This creates a continuity of purpose, aligns stakeholders, and facilitates prioritisation.
Unlike a Gantt chart that drives work, roadmaps are a description of the Product Backlog at a point in time. As market and user research comes to the Product Owner, and future plans adapt to changing needs, the roadmap evolves to communicate to stakeholders the new forward plan.