The Term scalability can be broken down into two simpler words for deeper contextual understanding: Scale & ability; so scalability is the ability to scale. In more technical terms it is a property of technology that represents its ability to grow to its theoretic operational demand limit.
Scalability in the context of blockchain and cryptocurrency is the ability of a system to handle more users & more transactions while not slowing down at all.
A CarWash is not a scalable business, only so many cars can get cleaned in a certain amount of time on a certain amount of land. The growth potential here is not linear or exponential.
A social media site, on the other hand, is very scalable; in fact the more people that are on it, the faster it grows!