The colony rush strategy refers to a strategy where your main aim is to occupy as many high quality planets before the AI can do it. This will probably hurt your economy in the short term in several ways. Secondly, new colonies generally end up in the red, particularly if you build only factories, starports and labs. Thirdly and more directly such strategies often damage your economy due to rush builds of both factories and colonies, which may cause your economy to overheat particularly if you rush build factories on bonus tiles.