Definition: Horizontal products can be used by all sorts of people for many different purposes. A vertical product, by comparison, is meant for a specific set of use cases. Most horizontal products are just fancy data structures.
For more details, see Joel Spolsky, the founder of Trello discussing horizontal products.
Another concept that I want to point to here is just the idea of horizontal products having a low floor, wide walls, and a high ceiling. The basic idea is that it should be easy to pick up in a useful way (low floor), have a high potential skill level allowing advanced use cases (high ceiling), and have a lot of potential for horizontal growth while maintaining the same skill level (wide walls).
Low floor and high ceiling are easy to grasp as concepts. If a product has wide walls, then that means people can do a wide variety of tasks at any skill level. For example, on my YouTube channel, people show me how they use Roam. Every once in a while people surprise me with some super advanced use case that I don't yet have the skill to execute myself. More often, they'll show me a creative use of functionality I'm already proficient in, expanding my own use case further.