What does 1 Samuel 3:4 mean?

Samuel, a boy, is lying down—likely sleeping—in the temple of the Lord, not far from the place called the Holy of Holies or Most Holy Place (1 Samuel 3:1–3). This space is separated from the rest of the area by a veil. Inside, the ark of God or ark of the covenant sits (Exodus 27:20–21). By His grace, God allows His presence to rest on the ark in some mysterious sense as a blessing for His people (Exodus 25:10–27).

Samuel hears a voice call to him and immediately says, "Here I am!" It becomes clear that Samuel thinks it is Eli who has called him (1 Samuel 3:5, 6, 8), though the text reveals to us from the start that the Lord is calling Samuel. As the chapter progresses, it becomes clear this is a literal calling from the Lord for Samuel to step into his new role as a prophet in Israel.