Circumpolar refers to stars that circle around the North or South Celestial Poles. These have a declination high enough that they never set.
Which stars count as circumpolar depend on where your location is on the earth. For a person standing at the North Pole, all the stars would be circumpolar, and Polaris, the North Star would be permanently at the zenith. At the South Pole, a similar situation would exist, except for the fact that there is no bright star close to the South Celestial Pole. At the equator, no star would be circumpolar, and the North Star would be permanently on the horizon. The way to determine what stars are circumpolar for your location is to take your latitude and subtract it from 90 degrees. All the stars above that number are circumpolar and never set. For example, at 30 degrees north, all stars above 60 degrees north are circumpolar. At 50 degrees south, all stars above fifty degrees south are circumpolar. In most actual observational conditions, your results probably won't be quite as clear as that, since the ten or so degrees closest to the horizon may be cluttered up with light pollution, haze or simply object clutter.
In the Northern Hemisphere, at middle latitudes, some of the important circumpolar constellations are Ursa Major, Draco, Cassiopeia and Ursa Minor, where Polaris is found.