The computer uses Random Access Memory (RAM) to process what the user is doing as they are doing it. This includes multi-tasking, writing letters, editing photos or browsing the net etc. The more complex the task, the greater the demand on the available RAM to process the data without slowing down the computer too much. Whilst 4GB of RAM is ideal for everyday users, for serious gamers who need their computer to process large volumes of data for their fast moving displays, 16Gb or even 32Gb of RAM will prove a worthwhile investment.
Remember, RAM can easily be upgraded if required at a later stage.