Every pound of floor load has to find a column, and every column carries the full weight of every floor above it.
A floor slab distributes its weight into beams. Each beam carries load from its tributary width — the perpendicular strip of floor it supports. Beam reactions become point loads on columns, and columns accumulate load from every floor above. By the time you reach the foundation, an interior column on a 5-story building carries 5 floors worth of tributary load. Understanding this load path is the first step in sizing every gravity member in a building.
A gravity system has a hierarchy: the deck spans to joists/beams (shortest span), beams span to girders (longer span), girders span to columns (longest span), columns carry everything down to footings. Each level collects load from the one above. Understanding this hierarchy tells you which members carry the most load and which are most critical — it's always the bottom of the chain (columns, footings).
Tributary area is how you figure out how much load each member catches. For a beam, it's the tributary width (half the distance to adjacent beams on each side) times the span. For a column, it's the bay area. Interior columns on a regular grid carry a full bay of load; edge columns carry half; corner columns carry a quarter. The load per member = floor pressure × tributary area. Simple concept, but getting it wrong cascades through the entire design.