An arbor is a garden structure on which plants and vines can grow. Sitting under an arbor can help you cool off when you're strolling through a garden on a hot, sunny day.
You may have come across the word arbor in the context of a garden structure, like a trellis or pergola. However, if you've heard of the holiday known as "Arbor Day," you know that an arbor is also a tree. These two meanings come from two different roots: the wooden arch type of arbor comes from the Latin herba, "grass or herb," while the tree arbor comes directly from the Latin arbor. Simply enough, it means "tree."