Download Page Size refers to the total amount of file content that needs to be downloaded by the browser to view a particular webpage. This includes HTML, CSS, Javascript and Images, though can include a number of other file formats. Generally media files like images and videos are significantly larger than text files and make up the bulk of Download File Size, but also represent the largest optimization opportunity. An important distinction here is 'Download' vs 'Raw' file size. Modern web protocols compress files during transfer, meaning files are usually smaller to download than their actual or 'raw' size. So any manual optimizations you perform would be on the 'raw' file. Download Page Size is one of the biggest contributors to Page Load Speed, which can directly affect rankings, user experience and conversions.
It is important to ensure your Download File Size is as small as possible by removing unnecessary files and minifying and optimizing others. 5MB is a good metric to strive for, though modern websites are gradually increasing in size.