Infomercial salesmen constantly tell you to seize the opportunity to buy their product while you still can. Supplies are limited! After all, everyone will want that solar-powered puppy petter, so you'd better grab the opportunity and buy two!
You can seize all kinds of things. Seize the day if you are taking advantage of the present circumstances and enjoying life to the fullest. Seize the castle, if you are a feudal lord fighting with your neighbor. Seize the inspiration if you want to tell a moving story. Seize authority if you are overthrowing a small country. Avoid, at all costs, being seized if you are under zombie attack.