QUOTE ([JAZ] @ Oct 1 2009, 12:53)

encoder delay: When the beginning is not really at the start, but some samples later (one block,576 samples usually in mp3).
encoder padding*: The amount of audio at the end of the file which is not part of the original file.
So if one cuts encoder delay from the beginning, and encoder padding from the end, the file should have the same amount of samples than the original.
And yes, whatever file that does not have this information cannot be easily decoded gaplessly.
You can add the information to your existing files with Foobar2000, but the process is manual, and is only exact if the originals come from a CD (because a CD is divided in blocks, so the amount of samples need to be a multiple of the samples of a CD block).
* Not to confuse with mp3 padding, which is just about adding unused (usually zeroed) portions of data between frames of the mp3 file.
why would you want the beginning to not be at the start?