A trellis is a type of structure used to guide and support climbing plants such as ivy.
Trellis is related to words for woven fabrics, and that can help you remember how a trellis looks: the material, usually a type of wood, is criss-crossed in a latticework. The purpose of the trellis is to support climbing plants, which can't stand straight like a tree or houseplant. With the aid of the trellis, the climbing plant wraps around the trellis and adds decoration to a garden. Sometimes trellis is used as a metaphor for other kinds of support.