This is a Latin term meaning “enclosed garden” — surprisingly useful in theology and literary criticism. The source of the trope is the Song of Solomon 4:12, which reads in the Vulgate: hortus conclusus soror mea sponsa hortus conclusus fons signatus; usually rendered in the language of King James, “A garden enclosed is my sister, my spouse; a spring shut up, a fountain sealed.” Have a meander over to Wikipedia for some interesting background I will not repeat here, including the idea of the hortus conclusus as the Virgin Mary, as well as the literal enclosed garden that became a commonplace of the Middle Ages.
Moving from theological to literary usage, the hortus conclusus could describe the Garden of Eden; then later, literary reflections of that Garden (e.g., in Milton’s Paradise Lost). Still later, gardens at further metaphorical remove. To situate the term in Inklings studies, think of the garden C.S. Lewis in the extreme west of Narnia in The Magician’s Nephew. Salwa Khoddam, of Oklahoma City University, has an excellent article on the subject, here.
In the world of Tolkien, think of the “enclosed garden” of Lothlórien in The Lord of the Rings; or the antecedent gardens in Valinor, where the Vala Irmo held court; or even the hortus conclusus in/of “Leaf By Niggle”. It’s a rich image, with a long history and a nimbus of wonderful connotations and associations. I am reminded, too, of Tolkien’s etymological riddle from The Hobbit:
An eye in a blue face
Saw an eye in a green face.
“That eye is like to this eye”
Said the first eye,
“But in low place,
Not in high place.”
Gollum’s answer — “Sun on the daisies it means, it does” — recalls the hortus conclusus in which Gollum, with his grandmother, was nurtured in his youth. And a daisy, of course, is just the day’s eye. The “enclosed garden” of Gollum’s childhood was situated in the neighborhood of the Gladden Fields, along the Anduin River, which Tolkien tells us in his “Nomenclature” refer to the fields of irises (Old English glædene).