Electron makes use of the main process as a sort of a back-end (you might call it "server side" as you do) and an entry point of your application. As you probably understand the main process can spawn multiple instances of
BrowserWindow, which are actually separate operating system windows each hosting a Chromium rendered web page run in a separate process called renderer process. You can think of renderer process as a simple browser window with potentially extended capabilities, like accessing Node.js modules (I write "potentially", because you can turn off Node.js integration for renderer process).