Muscle tone refers to residual muscle tension, which is tension that continues to reside in muscles while relaxed. The only way to gain muscle tone is to work out. This way you train your muscles to be tense even when your muscles are in a relaxed state.
You should also make sure that you don't injure your muscles during workouts, apparently this is very bad for muscle tone. Also remember lifting light weights many times do not help with tone as much as lifting heavy, despite what your personal trainer might say.
Cardio training is also important to muscle tone. Cardio training gets rid of all the access fat around your muscles making them have a "ripped" look.
For more in-depth information and details about how to gain muscle, see the related question and answer, listed further down this page, under Related Questions.
