The above Latin phrase means the faith which believes. The faith in your heart which grasps with firm grip all the Important doctrines that spring from the scriptures. It is a gift from the Lord, so that you may receive the other gifts that the Holy Spirit guides you to, especially being firmly convinced that Jesus Christ is your Lord and Savior from sin, death and devil, without any works to receive the gifts--they would not be gifts if you had to work for them. 

There is a similar Latin phrase that looks like this: Fides Quae, which means the faith which is believed, which are the teachings the Lord Jesus commanded us to teach (Matthew 28:19-20). This is why we teach confirmation classes to all ages. 

What is all this Latin mean? It helps us understand that it makes a difference what we believe, so that we do not follow cleverly devised myths (2 Peter 1:16), or fall into any errors that have crept into the Holy Church over the centuries.

We as Lutherans are a confessing church and that means that we hold to the ancient creeds which summarize from scripture what our faith is, and we stand on these teachings just as the Apostles did, as they were taught them by Jesus Himself--throughout His three-year ministry prior to His Passion; and after His resurrection, in the forty day period before His ascension into heaven. A person may say to you, "I believe in Jesus," but at that point you may want to know what sort of Jesus they believe in. The Holy Church spent the first six centuries following the resurrection of Jesus wrestling with that particular issue. And from the crucible of grinding out the heresies, the Church formulated credal statements to help believers grasp key elements of the faith delivered to them. 

That is why we confess almost every Sunday one of the three Ecumenical Creeds: Apostles', Nicene and Athanasian. In their respective order they became successively longer each time because the heretics tried to outmaneuver the Church in declaring on one hand their accuracy but on the other hand denying the Lord Jesus His Humanity or Divinity. I have said too much, but I think that at such a time as Christmas we should be clear as to what sort of Jesus we believe, teach and confess: True God, True Man, Savior by substitution of all humanity to pay for all sin and to satisfy the wrath of His own Father in order to cover our sin--becoming sin--so that we might by his righteousness become  the righteousness of God. (1 Corinthians 5:21)

Check out the largest Creed here: file:  https://bookofconcord.org/three-universal-or-ecumenical-creeds/

Jesus bless you as you confess Him with the faith given you to do so.