A service is scalable if it results in increased performance in a manner proportional to resources added.

Generally, increasing performance means serving more units of work, but it can also be to handle larger units of work, such as when datasets grow.

If you have a performance problem, your system is slow for a single user.
If you have a scalability problem, your system is fast for a single user but slow under heavy load

Horizontal scaling

Resource

Youtube - ByteByteGo – 7 Must-know Strategies to Scale Your Database