It says that when he appears, believers will be like him because they will see him as he is. The statement is connected to future hope. See 1 John 3:2.
In 1 John 3:1–3 the writer pauses to marvel at the Father’s love shown in calling believers “children of God.” This new identity explains why “the world” does not recognize them: it failed to recognize God’s Son. Yet the writer adds a forward-looking note: believers are already God’s children, but their future state is still hidden until Christ appears. This paragraph continues the thread from the preceding section about being “born of” God and practicing what fits that family identity (2:28–2:29). The writer shifts from instruction to wonder (“Behold”), then back to explanation: the world’s ignorance of believers follows the pattern of its ignorance of Christ.
This paragraph ties God’s love to a new family identity: believers are called “children of God,” and the writer treats that as a real present status (“now we are children of God”). That identity helps explain a social mismatch: “the world” does not recognize believers, and the reason given is that it did not recognize “him” first.
A careful answer keeps the immediate passage, the book's argument, and the whole-Bible storyline together, so the question is answered in context instead of as a detached idea.
Start with 1 John 3:2, then read the surrounding section so the answer stays connected to the passage's flow.
2Beloved, now we are children of God, and it is not yet revealed what we will be. But we know that, when he is revealed, we will be like him; for we will see him just as he is.