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It means your server or virtual server will be connected to the Internet via a dedicated 512Kbps connection.
Advantage: Unmetered Data Transfer (bandwidth), a 512kps line can push out over 87GB of data each month (running flat out 24/7.)
Disadvantage: The fastest any data can be transfered from your server is 512kbps (thats 64KBytes/sec.) If two people connect and simultaneously download a file each person would get 256Kbps (32Kbyte/sec) each.
Basically a fixed pipe is good if you have lots of people downloading small files, (web pages) but you'll need a larger pipe if you expect people are going to download large files (video/audio files) simultaneously.
Hope that helps.