Technically, the term "lean mass" is your mass with 0% body fat. This means your organs, blood, tissues, bones, muscle, everything. It does not include any fat whatsoever.
Being toned is just having a low body fat percentage. But "adding lean mass" means adding muscle without fat--hence the "lean" part.
But lean mass is not your weight at a low body fat percentage. And people with different body types may carry different amounts of lean mass, based on bone size and muscle mass. For example, I weigh approximately 193 and am 17.5% body fat or a little lower. It sounds like a lot, me being 5'9, but I carry around 155 pounds of lean mass. That means if I was 155 (my suggested BMI weight), I would look incredibly sick and old. And probably would be in the hospital. But no matter how much you weigh, you can look completely different. For example, 193 at 30% body fat is a completely different size from 193 at 15% (my goal).
So just something to think about!

The only way to build lean mass is to lift weights, and the only way to reduce body fat is through weights, cardio, and (of course) diet.