Single page applications or SPA's are at the time of writing this document one of the most in demand web application types and the basic idea behind the application style is to make a web application feel like a native application.
You can read more about SPA's here.
React is at the time of writing this document the most in demand SPA framework and although it is not the only one it is by far the most developed and well established.
You can read more about React here.
The following is a list of the most core concepts for React development and you are likely to find each of these in most projects.