Cloud Horizontal Scaling refers to provisioning additional servers to meet your needs, often splitting workloads between servers to limit the number of requests any individual server is getting. In a cloud-based environment, this would mean adding additional instances instead of moving to a larger instance size.
Cloud Vertical Scaling refers to adding more CPU or memory to an existing server, or replacing one server with a more powerful server.
Reference:
https://cloudcheckr.com/cloud-cost-management/cloud-vs-data-center-what-is-scalability-in-cloud-computing/ Horizontal scaling means that you scale by adding more machines into your pool of resources whereas Vertical scaling means that you scale by adding more power (CPU, RAM) to an existing machine.
An easy way to remember this is to think of a machine on a server rack, we add more machines across the horizontal direction and add more resources to a machine in the vertical direction.

With horizontal-scaling it is often easier to scale dynamically by adding more machines into the existing pool - Vertical-scaling is often limited to the capacity of a single machine, scaling beyond that capacity often involves downtime and comes with an upper limit.