A Gold Master is the copy of something that will be used to create all other copies. The concept comes from physical media which can be copied through physical processes (for example, stamping a CD, although the term is much older than that). The Gold Master is the original copy. A negative "cast", called a matrix, of it is taken, and this matrix is used to produce a number of working masters. These masters are what are used to produce the working matrices which will be used to produce the actual CDs.
So, when you get a copy of the Gold Master, it indicates that the publisher is confident that the software won't change any more, and is willing to start duplicating physical media*, along with the capital outlay that implies. Unless an absolutely show-stopping bug is found, the Gold Master will be the initial release version, as changing it would require recall, destruction and replacement of any media already produced. Any bugs that are found at this stage would be fixed in the *.1 release.
*Apple aren't going to be creating many Lion DVDs, but they will be creating and installing the disk images for their computers that will ship with Lion, and they won't want to have to re-do them.