It means to be persuaded that He is God; to have confidence that when He died and rose again He rescued you from satan's influence. It means that you are convinced you have been forgiven and reconciled with God.
But that's just the beginning.
To believe in Jesus is not a one-time, once-and-for-all happening.
Believe is a verb that implies continual action. To believe means to constantly have confidence, to constantly be committed, to constantly have faith and trust in Jesus to provide all you need.
This kind of belief leads to a deepening relationship.
But there's still more.
Believing also means knowing without a doubt that God speaks to His children through His word, the Bible. If we believe in Jesus, we will believe what He said; we will follow Him and do what He did.
When talking with His closest disciples Jesus said "Remain in me, and I will remain in you. For a branch cannot produce fruit if it is severed from the vine, and you cannot be fruitful apart from me." (John 154-NLT)
Remain in Jesus. Live with Him daily. Study and learn. Put what you learn into practice wherever you go and whatever you do.
As you live with Jesus daily you will get to know Him intimately. As you experience His love, compassion and care you will be more committed to Him and you will learn to trust Him with every area of your life.
Finally, believing in Jesus means assurance that when you step out of time into eternity He will be there to welcome you, to rejoice with you and to introduce you to the wonders of heaven.
To believe in Jesus means to live in close relationship with Him all day, every day. It is an exciting and rewarding journey through this life and the promise of a secure and eternal future with Him.