A necktie is a long piece of cloth that people, usually men, wear around their necks as part of their clothing. It goes over the collar of a shirt and usually reaches down to the middle of the chest.
You know how you tie your shoelaces in a bow? Well, tying a necktie is kind of like that, but a bit trickier. The two ends of the tie start apart from each other and you have to bring them together by crossing one over the other, then looping it up and over the other, and tucking it down under the loop. Then you adjust the knot and make it look neat and tidy.
Neckties come in many colors, patterns, and styles. Some people wear them to work, to weddings, or to other fancy events. They can make a person look more formal, serious, or stylish. But don't worry, you don't have to wear one if you don't want to. It's just another option for dressing up.