John 8     ² Now early in the morning He came again into the temple, and all the people came to Him; and He sat down and taught them. ³ Then the scribes and Pharisees brought to Him a woman caught in adultery. And when they had set her in the midst, ⁴ they said to Him, “Teacher, this woman was caught in adultery, in the very act. ⁵ Now Moses, in the law, commanded us that such should be stoned. But what do You say?” ⁶ This they said, testing Him, that they might have something of which to accuse Him. But Jesus stooped down and wrote on the ground with His finger, as though He did not hear.

⁷ So when they continued asking Him, He raised Himself up and said to them, “He who is without sin among you, let him throw a stone at her first.” ⁸ And again He stooped down and wrote on the ground. ⁹ Then those who heard it, being convicted by their conscience, went out one by one, beginning with the oldest even to the last. And Jesus was left alone, and the woman standing in the midst. ¹⁰ When Jesus had raised Himself up and saw no one but the woman, He said to her, “Woman, where are those accusers of yours? Has no one condemned you?”

¹¹ She said, “No one, Lord.”

And Jesus said to her, “Neither do I condemn you; go and sin no more.”

Here we have the whole passage that you are asking about. The problem was that the scribes and Pharisees were trying to find something against Jesus, which would get Him into trouble and get Him out of their way. This whole thing was obviously staged, as the Bible says that if there is adultery, both parties would be liable for stoning. But here they only have the woman, and have let the man go free, which shows that it is a setup. The woman is obviously guilty, caught in the act, and they want Jesus to say what He thought. If He said, stone her, she’s guilty, they could go to the Roman authorities and accuse Him of going against Roman law, which forbade the death penalty from anyone but the authorities. But if He said, let her go, they could then accuse Him of not upholding the law against adultery. 

Jesus obviously knew their plan, and so He says nothing, just writes in the dust, probably listing the sins of all the accusers. And when they press Him for an answer, He says that the one who is without sin, should throw the first stone. With their sins written in the dust, no-one could pick up a stone, and they all left. Jesus also knew the heart of the guilty woman, and did not condemn her, asking her to turn away from her sin. That is exactly what God does to us, He does not condemn us, but wants us to turn away from our sins, and give our lives to Him.

John 3:17   “For God did not send His Son into the world to condemn the world, but that the world through Him might be saved.”

1 Timothy 2     ³ For this is good and acceptable in the sight of God our Savior, ⁴ who desires all men to be saved and to come to the knowledge of the truth.

2 Peter 3:9   The Lord is … not willing that any should perish but that all should come to repentance.

Many scholars believe that this woman is Mary Magdalene, and that this action by Jesus was the turning point in her life. What we should understand, is that God is trying His best to get us into heaven, not keep us out, and that He is willing to forgive every sin. Not one of us is without sin, so it is impossible for anyone to cast the first stone at anyone else.

I trust that this answers your question. God bless!