Xylem is a channel within the stems/trunk of a plant that draws water up from the lower parts of the plant (where water entered via roots) to higher parts of the plant.
Xylem can transport water, and also different minerals that are soluble in water, against the pull of gravity, so that all parts of the plant can stay healthy and nourished, even though much of the plant's water intake is happening below-ground.
Xylem is technically a tissue, and can be found throughout the plant and differ in appearance and structure depending on the type of plant it is in. Water is able to defy gravity due to the polarity of the water molecules, which creates a weak attraction between them. As water is lost from the leaves in the process of transpiration, liquid water is pulled upward to replace the previous water molecules, and all subsequent molecules are also pulled up in a phenomenon called "capillary action."
This can be modeled by you placing the lower end of a rag in a bowl of water while holding the top part of the rag. In a matter of minutes, you will notice that the top part of the rag is starting to get wet even though only the lower part of the rag is touching the water!
Xylem is different from phloem, another transport tissue in plants, because xylem primarily transports water, while phloem transports a number of other molecules, such as the products of photosynthesis in the leaves. As such, xylem has a one-way flow (upwards), while phloem can pump materials downward to other parts of the plants as needed.