Dead load is the load that never goes away: the permanent weight built into the structure itself. Design codes give it its own symbol — G for permanent actions in Eurocode 0, D in the ASCE 7 combinations — precisely because permanence changes how it is treated. A load that is always present is known with more confidence than one that comes and goes, so the codes multiply it by smaller factors: Eurocode 0 pairs 1.35G with 1.05Q in equation 6.10a and 1.15G with 1.5Q in 6.10b, while the basic ASCE 7 LRFD combination is 1.2D + 1.6L + 0.5(Lr or S or R).
Sometimes dead load helps rather than hurts — it anchors a structure against wind uplift, which is why the ASCE 7 ASD wind combination D + 0.6W keeps the dead load unfactored. Prescriptive residential tables fold a standard allowance straight into the lookup: the IRC deck tables are computed for a 10 psf dead load on top of the 40 psf live load, and every deck post load in the footing calculation is taken at the combined 50 psf.