If you have poise, you are cool under stress. People with poise can handle pressure without showing it.
If you’re calm while singing the national anthem in front of thirty million people, you’ve got a lot of poise. This is a rare quality people admire. Another meaning is to brace yourself for something difficult, as in: when you hear about a tornado, you're poised for more bad news. You can also be poised as in "on the brink" of something. If you stand on the diving board it could be said you’re poised to take the plunge.