This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
橪 combines 木 (tree) with 然, which likely contributes sound. The exact historical development is uncertain.
The tree radical 木 beside 然 suggests a type of tree or plant, but the exact meaning is unclear.
For ゼン, imagine a Zen monk sitting under the tree 木: Zen -> ゼン, and the tree is 橪.