A web application is a software application accessible via a web browser and operates over the internet. Unlike traditional software installed on a local computer, a web application runs on a remote server and is accessed through the user's browser.

Web applications can encompass a wide range of functions, from simple interactive pages to complex applications such as social networks, email services, online stores, productivity tools, and more. They often use a combination of different technologies like HTML, CSS, and JavaScript on the client-side (in the user's browser) as well as backend technologies like databases, server-side scripting languages (e.g., Python, PHP, Ruby), and frameworks to support functionality.

Accessing web applications via the browser makes them platform-independent, allowing them to be used from various devices with an internet connection—be it a computer, tablet, or smartphone.

Server-Side Rendering (SSR) is a process where web pages or web applications are rendered on the server before being sent to the browser. In contrast to traditional client-side rendering (CSR), where the browser receives the code and handles the webpage's rendering, SSR involves a significant portion of rendering taking place on the server.

The process of Server-Side Rendering operates as follows:

1. Requesting a Web Page: When a user requests a web page, the browser sends a request to the server for the corresponding page.

2. Server-Side Rendering: The server receives the request, processes it, and renders the HTML page with all the necessary content and data.

3. Transmission to the Browser: The server sends the fully rendered HTML page to the user's browser.

4. Interactivity: Once the browser receives the HTML page, it displays it immediately while simultaneously loading JavaScript and CSS files. These files enable interactivity on the webpage by adding additional functionalities or enhancing the user experience.

The primary advantage of Server-Side Rendering lies in the quicker display of content to the user, as the browser receives a complete HTML page that can be displayed while other resources are loading. Additionally, SSR also offers benefits in terms of Search Engine Optimization (SEO) as search engines can better index the page's content when it's provided directly as HTML.

SSR is commonly used for complex web applications, content-centric pages, and pages that require better SEO performance. However, it's not always the best choice for every application, as it can cause additional server load and might not be necessary when an application primarily consists of interactive components that can be rendered on the client-side.