Managed Code and Application Domains

Question

Actually i downloaded a PDF file from this website and i was reading the topic called "managed code" and i found it very interesting. But i got stuck at a paragraph which is as follows:

"Application domains, a concept introduced in .NET, permit multiple applications to run in a singleprocess. Before managed code runs, it goes through a verification procedure to establish that it will
not access memory or perform an action that would cause the process to crash. If the code passes
this verification, it is said to be type safe. Type-safe code allows the CLR to define application
domains, which carry the same level of isolation and protection as a process. While not totally free
of charge, interapplication communication, across application domains, avoids most of the
performance penalties associated with interprocess communication."

The problem is, i found it very difficult to understand from the second line of the paragraph.. Please help me undertand this by giving a simple example since i am a beginner!!Also if possible, please give me some links to understand more about Managed Code!! Hope to get some good replies.. Thanks in advance....