间 sets 日, the sun you already know, in the gap between the two leaves of a door 门 — a sliver of daylight slipping through where the doors don't quite meet. That thin strip of light was never fully inside or outside, so it gave Chinese the very idea of "in-between": a gap, a room, the space that separates one thing from another. An older, closely related character, 閒 — door plus moon instead of sun — drew moonlight in that same gap and came to mean idle, unhurried time; that sense survives today in 闲 (xián), "free, at leisure."
A piece of culture
The world of 人间 人间 (rénjiān), literally "among people", is the classical word for the human world — the space beneath heaven where people actually live out their lives. The Song dynasty poet Su Shi closed his most famous verse, written for the Mid-Autumn Festival in 1076 while missing his brother, with a wish that reaches straight across that gap: 但愿人长久，千里共婵娟 — "May we live long, and share this beautiful moonlight, though a thousand li apart." You already know 天 (sky, heaven) from chapter three: 人间 is everything that unfolds beneath it.
Use it in a real word
中间
zhōngjiān · zung1 gaan1
middle; in between — literally "center-between"; you already know 中