"lower density" generally refers to a substance or material that has less mass per unit volume.

Density (ρ) is defined as the mass (mm) of an object divided by its volume (V):

ρ=m/V​

If an object has lower density, it means that it has less mass for a given volume or occupies more space for a given mass.

Conversely, higher density implies more mass packed into a given volume.