When a person performs Netilat Yadayim (ritual hand washing), whether in the morning or before having a meal with bread, they have to be careful that there is no Chatzitza on their hand. Chatzitza means an interposition or something that blocks the water from reaching all the different parts of the hand. Chatzitza is defined as either something which covers the majority of the spot that you don't want it to be on—in this case, the hand. If the majority of the hand is covered with dirt or mud or something else, then it is automatically a Chatzitza, and the person would have to clean it off before performing Netilat Yadayim properly.
But a Chatzitza can also be something covering even a minority—even a small part of the hand—if the person is makpid alav, meaning that he cares to want to remove it. If a person has dirt on their hand that they would not otherwise leave there, they would normally clean it off. Or a woman might have nail polish that she wants to remove to fix it and make it look appropriate. That would be considered a Chatzitza, and the person would have to remove it before performing Netilat Yadayim. If the person washed their hands with that Chatzitza on, they would have to wash again. The Netilat Yadayim would not count, and they would have to say a Beracha (blessing) again.
A good example of a Chatzitza is a person who wears a ring on their hand. If the person takes off their ring for other activities, such as a woman who takes it off to cook or bake or the like, then it is considered something they are makpid on, and they would have to remove it for Netilat Yadayim. If the person, however, never removes the ring—as happens with many men, who leave it on all the time, whether they play sports or shower—then it is considered part of the hand and does not constitute a Chatzitza me'ikar hadin (according to the letter of the law). But many are machmir (stringent) and remove it anyway.