Physical Memory Usage (RAM) is the actual memory allocated for your account. Virtual memory is usually a file on a disk drive that the operating system uses to store information (swap-to-from) when the real memory becomes full, for instance, the page (swap) file on a Linux system. Therefore, if you try to publish a big post, it might take all physical memory to do so, but after some time it will be normalized.
If this value reaches the limit you may begin to experience PHP errors (if applicable) on your website, or in very extreme cases may see a CloudLinux error page. These errors are typically only brief and once the usage has reduced to below the limit, will automatically clear.