The inspiration of Scripture is a reference to the divine authorship of Scripture — the Bible has God as its singular source and authority. It comes from God.
Perhaps no theologian wrote as carefully or extensively on the topic of inspiration as B. B. Warfield. In his classic work, Inspiration and Authority, he writes this about the word “inspired by God” (theopneustos) in 2 Timothy 3:16 —
The Greek term has, however, nothing to say of inspiring or of inspiration: it speaks only of a “spiring” or “spiration.” What it says of Scripture is, not that it is “breathed into by God” or is the product of the Divine “inbreathing” into its human authors, but that it is breathed out by God, “God-breathed,” the product of the creative breath of God. In a word, what is declared by this fundamental passage is simply that the Scriptures are a Divine product, without any indication of how God has operated in producing them. No term could have been chosen, however, which would have more emphatically asserted the Divine production of Scripture than that which is here employed. The “breath of God” is in Scripture just the symbol of His almighty power, the bearer of His creative word. “By the word of the LORD,” we read in the significant parallel of Psalm 33:6 “were the heavens made, and all the host of them by the breath of his mouth.” And it is particularly where the operations of God are energetic that this term (whether רוח, ruach, or נשמה, neshamah) is employed to designate them—God’s breath is the irresistible outflow of His power. When Paul declares, then, that “every scripture” or “all scripture” is the product of the Divine breath, “is God-breathed,” he asserts with as much energy as he could employ that Scripture is the product of a specifically Divine operation.