The top and bottom components at the head tube of the bike are where the bearings are installed to assist in smooth steering. They connect the fork steerer tube to the frame and facilitate the smooth rotating of the handlebars.
They may or may not. There are loose-bearing and sealed cartridge varieties. They must be installed properly—compress the bearings but not too hard—to prevent play or resistance when the handlebars are turned. Sealed headsets are very low maintenance and function well in wet and muddy conditions and are an ideal choice for extended rides. A quality, light headset can reduce steering mistakes and front-end drag on long trips.
