This pharse refers to moving up in rank within a particular field or business. If someone "climbs the corporate ladder" to the top, they started out at the bottom (
stockboy, mail clerk, etc) and ended up at the top (CEO,
CFO, vice
pres., etc.)
Tim lived
the American Dream. He started out as a desk clerk and climbed
the corporate ladder to the top. He is now a CEO in the same field that he had started out in.