Density is mass divided by volume, so knowing any two of the three gives you the third. Density, mass, and volume are three sides of the same relationship - know any two and the third follows. A plain unit converter can turn kg into lb, but it can't tell you what 200 litres of water, or half a cubic metre of concrete, actually weighs. This can.
Density is the relationship that connects how much space something takes up to how much it weighs - and it's genuinely different for every material, which is exactly why a generic unit converter can't help here. A litre of water weighs one kilogram. A litre of concrete weighs about 2.4 kilograms. Same volume, very different weight, because the material is different. Pick any two of density, mass, or volume and the third is determined - this solves for whichever one you're missing.
The material presets are genuine reference values, not guarantees - real density varies with moisture, temperature, compaction, and exact composition. Topsoil especially can weigh 20% more when wet than dry. Treat these as a solid starting point for planning, and use the exact figure on your specific material's spec sheet when precision actually matters (structural work, shipping weight limits).
Density is mass divided by volume. A 2 kg object occupying 0.5 litres has a density of 4 kg per litre.
Multiply the volume by the density of the material. One litre of water has a mass of about 1 kg; one litre of steel is closer to 7.85 kg.
Each of these has its own page with the exact factor, a reference table and the working shown. The converter above handles any of them too - these exist so the specific answer is one click rather than a search.