Without going into a long scientific explanation about how and why electrons are attracted to and repelled by atoms, just accept that if a nitrogen atom has 3 hydrogen atoms attached to it is bad, if it has 4 hydrogen atoms attached, it is good.
The pH (power of Hydrogen) level in your tank determines how many of the ammonia molecules are toxic or non-toxic. If your tank water pH is at 8.3, most or all of the ammonia molecules will be the NH3 (toxic) variety. If your pH is 7.5, the same amount of ammonia in your tank will be 1/5th as toxic because about 4 out of 5 of the ammonia molecules will be the NH4 (non-toxic) variety.
Some people make the mistake of performing a partial water change to reduce ammonia levels during the cycling process. Normally, when ammonia levels go up, the pH drops at the same time. By performing a partial water change, the total ammonia levels may drop slightly, but the pH will also rise (the buffering effect of new saltwater), increasing the toxicity of the remaining ammonia. A safer method to reduce the ammonia levels would be to use an ammonia neutralizing product such as Amquel, then perform a water change to "freshen" the water, if you wish.
When adjusting your pH level, whether up or down, it is important to do it slowly. If the pH is changed too rapidly, your tank critters can suffer from "pH shock", which can be fatal. The more sensitive animals in your tank may not tolerate a pH shift of more than 0.5 units per day, however most fish can handle a 0.5 shift in a couple of hours without any problems.