In hardwoods they are one of two types of Xylem plant tissue, the other being vessel elements. Softwoods are lacking vessel elements, and thus tracheids are the only cell type involved in water transport (and support) in these trees.
Tracheids have two major functions, structural support and transport of water and minerals collected by the roots. They occur as rigid fiber bundles throughout the plant and provide resources to leaves and other parts of the plant (stem, flowers, fruits etc.).
The presence of tracheids (and other water-conducting cells) is the defining characteristic of vascular plants to differentiate them from non-vascular plants. In seedless vascular plants and gymnosperms, they are the most important water-conducting vessels. In angiosperms, they serve a lesser role as wood vessels (vessel elements) have largely taken over the function of conducting water.