The sentry can cast force field, a nigh impregnable barrier that prevents all non-massive units from entering the area it blocks (ultralisks, colossi and thor will walk right through, however). This is one of the most versatile abilities in the game, and can be used to defend effectively against an early ground rush, slow down enemy resource income, or turn almost any situation into a tactical advantage for the protoss. Its proper usage becomes more critical as the match progresses.
Many early to mid-game pushes are virtually impossible to stop without properly using this ability. For example, a four warp gate rush will hit your base before you have any substantial defenses, unless you scout aggressively for enemy proxy pylons or also go the four gate route. Force field, however, will allow you to block your ramp and stall for as long as your sentries have energy to sustain it (provided you remain attentive of your ramp or choke point). Certain pushes with zerg banelings or terran marines with stimpacks will also require good use of the force field to repel.
In the late stages of the game, force fields will be instrumental in turning the tide of battle to your advantage. They can prevent units from escaping, push back the ranks of your enemy's ranged units, prevent melee units from clashing with your army, or enclose enemy units entirely; leaving them ripe for the slaughter. There are lots of neat things you can do with force fields. Be sure to watch videos or replays of professional games and, more importantly, experiment and practice!