If you know a bit about software and operating systems (OS), you’ve heard about “processes”. Modern operating systems, such as Linux or Windows, will run your software applications inside separate processes. This is an OS technique for isolating different software. For example you can make sure that whatever your web server is doing – it cannot touch the memory of your database server; and vice versa.
This sounds like a good thing for security, and it is. It is also essential for reliability and stability. Bugs in one software application will less often cause trouble in other software applications, even if they are running simultaneously in the same physical memory, due to being separated into different “processes”.