Japanese cypress
桧 refers specifically to the Japanese cypress tree, a type of evergreen conifer valued for its wood and often found in traditional Japanese architecture and gardens.
桧 combines 木 (tree) with 会, which likely contributes the sound カイ. The character was created to name the Japanese cypress, a specific tree species.
A tree 木 that meets (会) the eye: the Japanese cypress stands out with its straight trunk and fine wood, like a tree you'd recognize at a meeting.
For カイ, imagine a cypress tree by the sea (海, かい): the sound of the waves カイ helps you remember the reading, and the tree grows near the coast.