inside: on the interior of a building, or going in there, as opposed to outside
This is a small word with one job: it tells you that something is on the inside. It does not name a room, and it does not label one of two spaces as the inner one; the word for that is פְּנִימִי, "inner." It simply marks the inside, and it is regularly set against the word for what is outer (חִיצוֹן) in the same breath. Solomon overlaid the floor of the temple with gold "inside and outside" (1 Kings 6:30).
It is built from the ordinary word for "face" or "front" with an ending that means "toward," so it can say where something sits or which way it is going, and the two shade into each other. Most of its uses describe a building, and above all the temple: cedar lining the walls inside, gold laid over the interior, windows set on the inner side of a gateway, the inner room made ready for the ark. Where the sentence has a verb of movement, the same word carries a person or an offering across the threshold and further in.
That threshold is often the whole point. Blood that should have been brought inside the holy place was not (Leviticus 10:18); priests go inside the temple to carry out what does not belong there (2 Chronicles 29:16); a guide walks further in to measure the innermost room (Ezekiel 41:3). Inside, here, is the space not everyone may enter.
One thing this word almost never does is turn inward toward a person. The Greek words for "inside" that stand in for it (ἔσω and ἔσωθεν) readily describe the hidden self, the heart underneath the surface. The Hebrew word stays with rooms and thresholds, and even in Psalm 45:13, where the king's daughter is "all glorious within," the likeliest picture is a woman indoors in the palace rather than a comment on her character.