TO provide evidence. To show that something actually happened, and that you have proof for it. I have to answer the same question for an essay. Hope this answer helps!
What Does It Mean To Bear Witness?
Romans 8 and verse 16 or so Says: The Holy Spirit himself bears witness with our spirits that we are children of God. ( This can only happen in the Spirit indwelt christian, because he who does not have the Spirit of Christ dos not belong to God)