Razor syntax is a simple programming syntax for embedding server-based code in a web page. In a web page that uses the Razor syntax, there are two kinds of content: client content and server code. Client content is the stuff you're used to in web pages: HTML markup (elements), style information such as CSS, client script such as JavaScript, and plain text.
Razor syntax lets you add server code to this client content. If there's server code in the page, the server runs that code first, before it sends the page to the browser. By running on the server, the code can perform tasks that can be a lot more complex to do using client content alone, like accessing server-based databases. Most importantly, server code can dynamically create client content — it can generate HTML markup or other content on the fly and then send it to the browser along with any static HTML that the page might contain. From the browser's perspective, client content that's generated by your server code is no different than any other client content. As you've already seen, the server code that's required is quite simple.
ASP.NET web pages that include the Razor syntax have a special file extension (.cshtml or .vbhtml). The server recognizes these extensions, runs the code that's marked with Razor syntax, and then sends the page to the browser.