The term associated with the ‘believe in Jesus’ is stated in the creed, a profession of faith in the Catholic Church:
We believe in Jesus Christ, His only son, our Lord…
and so on and so forth. It is said in the Nicene form every week at Mass. For devotions and prayer, such as the rosary, a form called the Apostle’s Creed is used. They both profess the belief in Jesus Christ.
“Believe in Jesus” means that we acknowledge that Jesus existed, and everything associated with him, (he is God, he died for our sins, he was risen from the dead etc.). That is it on the surface.
A deeper view is that not only do we accept his existence and nature, but the part that nature takes in our lives, for Scripture states he is, ‘The Way the truth and the life’. (John 14:6).
It also means:
To believe in God
Because Jesus is the Lord, and he is God. It also means we acknowledge who God is:
Love (1 Jn 4:16)
Truth (See above, John 14:6)
All that is good (I can’t remember the references)
To believe in Christ is also to believe in the fulfillment of the old covenant, and accepting the new (Magnificat).
For a more in depth view, I suggest reading the entirety of Scripture and the Catechism of the Catholic Church, that should give you a pretty good idea of what it means to believe in Jesus
