Both are indicators of the chemical composition of silicate minerals, magmas and rocks.
Mafic is used for silicate minerals, magmas and rocks that are composed of heavier elements, like magnesium, iron, calcium and sodium. They are dark and have high specific gravities.
Opposite to the this, felsic silicate minerals, magmas and rocks in their composition have less from these heavy elements. They are composed from silicon, oxygen, aluminium and potassium, are light in color and have lower specific gravities.