Increasing a system's capacity by adding more instances or nodes (more identical servers). This is the preferred strategy in cloud architectures for its higher availability and fault tolerance.
Fundamental in SAA-C03 and AZ-104. Auto Scaling Groups in AWS and VM Scale Sets in Azure implement automatic scale out. Typical question: scale out or scale up for high availability? Answer: scale out.
Horizontal scaling (Scale out) is a concept that can show up on the exam. Check the full guide to see it in context.