A scalable website is one that can serve an increasing number of concurrent users with minimal impact to latency or functionality.

First, you should address basic performance considerations that are relevant to any website/web application. This includes caching frequently requested data, optimizing database queries to avoid unnecessary table joins, ensuring your hardware configuration is optimal for the read/write profile of your application, and profiling your code to ensure you have not accidentally introduced pathways that are unnecessarily executing in quadratic time (or worse).

Beyond these basic performance points, there is also vertical and horizontal scalability.

"Vertical scalability" mostly refers to increasing the performance of a single piece of hardware to serve more users. This is mostly independent of the underlying website/web application, so I will assume you are not interested in that.

"Horizontal scalability" mostly refers to whether or not you are able to add (or remove) additional hardware/virtual cloud services in response to increased/decreased demand. 

Achieving horizontal scalability is not simple. Here are some examples of options for increasing horizontal scalability:
1) load balancers
2) database replication and sharding
3) containerized environments
4) microservices
5) parallelized execution

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