What is the main idea of the era of colonization?

There isn't one. Colonisation went through successive stages, sometimes shaped by policy, sometimes an improvised response to geopolitical conditions. At various stages it was driven by military or religious zeal, individual or collective greed, considerations of national prestige, strategic competition among the powers, notions of transplanting surplus metropolitan population, promotion or protection of national commercial interests against rivals, or misplaced doctrines of racial superiority or a "civilising mission". And sometimes it was a response to unforeseen consequences of the powers' unplanned actions. Colonisation took different forms and had no overarching ideological foundation, like much in history that on the surface seems consistent but on closer investigation falls apart as a coherent process.