It means you have repented of all your sins, you have asked Jesus into your heart, you have chosen from then on to take up your cross and follow Jesus daily, you have accepted Jesus as your saviour. You have opened the door of your heart fully to Jesus, the door He was knocking on for some time.
It means you are born again, the Holy Spirit cannot enter your heart unless you come to Jesus, come to the cross and allow your old self to be crucified, so the new self, the new creation can go forward, practicing righteousness, worshipping God, Praising Jesus.
Bit this is just the start, the Spirit enters our heart, we no longer belong to sin, the world and the devil, we are set free, free indeed.
God puts His spirit in your heart, he has basically connected you to Himself through Jesus, connected you into righteousness, you now have direct access to God through Jesus. The spirit of the Lord abides in your heart, but he wants you to abide in Him also through Jesus, read John 15, it is a wonderful passage, remember we are nothing without Jesus, He is our life given sap.
John 15:1-17 (NKJV)
1 "I am the true vine, and My Father is the vinedresser.
2 Every branch in Me that does not bear fruit He takes away; and every branch that bears fruit He prunes, that it may bear more fruit.
3 You are already clean because of the word which I have spoken to you.
4 Abide in Me, and I in you.
As the branch cannot bear fruit of itself, unless it abides in the vine, neither can you, unless you abide in Me.
5 I am the vine, you are the branches. He who abides in Me, and I in him, bears much fruit; for without Me you can do nothing.
6 If anyone does not abide in Me, he is cast out as a branch and is withered; and they gather them and throw them into the fire, and they are burned.
7 If you abide in Me, and My words abide in you, you will ask what you desire, and it shall be done for you.
8 By this My Father is glorified, that you bear much fruit; so you will be My disciples.
9 "As the Father loved Me, I also have loved you; abide in My love.
10 If you keep My commandments, you will abide in My love, just as I have kept My Father's commandments and abide in His love.
11 These things I have spoken to you, that My joy may remain in you, and that your joy may be full.
12 This is My commandment, that you love one another as I have loved you.
13 Greater love has no one than this, than to lay down one's life for his friends.
14 You are My friends if you do whatever I command you.
15 No longer do I call you servants, for a servant does not know what his master is doing; but I have called you friends, for all things that I heard from My Father I have made known to you.
16 You did not choose Me, but I chose you and appointed you that you should go and bear fruit, and that your fruit should remain, that whatever you ask the Father in My name He may give you.
17 These things I command you, that you love one another.
The Vine
The root, also called the lump, is Holy.
Jesus is the vine, the Holy sap runs through Him.
We are the branches, the first branches were the Jews, but as we know Jesus opened the door to us Gentiles also, to the whole world. We are the branches, we reply on Jesus the vine, without Him we are nothing. We are the branches, fed by the 'life giving' sap from the vine, but not for ourselves, the Blessing runs through us, we are to bear fruit, we are expected to bear much fruit.
If we don't bear fruit, we will be cut off from the Blessings.
v10 If you keep My commandments, you will abide in My love,
just as I have kept My Father's commandments and abide in His love.
The Holy Spirit abides in us, not for our benefit but that by us abiding in Jesus God will be Glorified.
The sap in the vine is the Holy Spirit, from God, through Jesus, to the branches SO THAT they will bear much fruit, bring Glory to God. If the Holy Spirit is not abiding in a persons heart they are Not born again.