抓住 (zhuā zhù) means to grab and hold, catch, or seize. 抓 means to grab or scratch and 住 here is a result complement meaning to hold firmly or to stop. Together they express the successful act of catching and retaining something: 抓住小偷 (catch a thief), 抓住机会 (seize an opportunity), 抓住要点 (grasp the key point). The 住 complement indicates that the grab is complete and the object is held. Contrast with 抓 alone, which just means to grab.