Scalability within the context of cryptocurrency refers to a network’s ability to support all the users that want to participate. For example, in the early days of the Internet, it took days for an email to send. The Internet had to “scale” or increase it’s speed and capacity so that email could be used by large numbers of people. When people started to attach photos to emails, the Internet could not handle so many large files, so it had to scale again. In 1993 nobody would have guessed that the Internet would ever reach a large enough scale to support an application like YouTube, but the Internet has proven to be a very scalable network over time.
Many people are working hard to make major cryptocurrencies such as Bitcoin and Ethereum scale.