Glutamine in many physiological processes, the most important functions are donating nitrogen to the synthesis of NEAA's, and donating carbon to to the krebs cycle (aerobic energy release), synthesising ammonia to balance pH, as a building block for neurotransmitter synthesis in the brain, and as a fuel for many cells of the immune and digestive systems. It also is a preferential fuel for intestinal bacteria.
More related to bodybuilding it can help slightly increase HGH levels, is a required structural amino acid for building muscle, can help increase the volume of muscle cells (which increases their stored energy potential and contractile strength), and increases the rate of muscle repair after damage.
Despite all the good stuff though it's important to remember that the body can synthesise it's own glutamine very easily, and due to the preferential uptake of L Glutamine by the intestinal cells, only about 30-40% of any you supplement with actually reaches the blood supply or muscles. If taken in a high enough dose however (5-10g minimum), this can still be enough to make a difference.
The most pronounced effect from supplementing with decent sized amounts of glutamine is improved digestion and intestinal health, normally in a very obvious way after two weeks or so. Specific effects on muscle gain are far less pronounced, primarily because the body (in times of good health) mostly makes the glutamine it needs for anabolic purposes in the very cells that need it, and at the same time uses glutamine obtained from diet more for non anabolic purposes such as production of ammonia.