First off, a Product roadmap is not an agile term. It's a generic term that can be used in several contexts. Widely used in the area of new product development, you can think of a Product Roadmap as a plan of how your product will be built over the course of time.
In practice, a product roadmap can be presented visually showing the spread of features on a timeline. For example, you might show the set of features that'll be delivered every quarter in a year. Your near-term plan would be more detailed and specific (e.g. Quarter 1 is near-term) whereas the long-term plan (e.g Q3 and Q4) can be an outline. You can think of this as an agile planning technique, commonly referred to as "rolling wave planning". Hence, when you're creating your product roadmaps, it'll be wise to consider your planning horizon, that is "how much of the product's future you can sensibly predict?"
Lastly, you must remember that product roadmaps are only "forecasts", i.e. predictions or estimates based on what is known at the current time and that they are subject to change based on market trends and various other factors.
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