is an artificial, non-saccharide sweetener, used by people that try to loose weight.
They want to prevent that they become fat by eating too much sugar (or they have other reasons).
Fact is that sugars contain a lot of energy. In stead of sugar in the coffee, we can take substitutes like aspartame; but there are others.
Many sugar containing products (for example jam and coca cola) have also their diet products, i.e. without sugar, but with the substituting sweeteners.

The secret of aspartame is that it leaved the body as it entered it, without participating in the metabolisme, without any energy impact, without any fat making.
But a warning is needed: some sugar substitutes can be carcinogeneous, not healthy.