It's best to look at the entire thought in a translation that isn't 500 years old.

(Luke 1:1-4 NIV) Many have undertaken to draw up an account of the things that have been fulfilled among us, {2} just as they were handed down to us by those who from the first were eyewitnesses and servants of the word. {3} Therefore, since I myself have carefully investigated everything from the beginning, it seemed good also to me to write an orderly account for you, most excellent Theophilus, {4} so that you may know the certainty of the things you have been taught.

It tells us that there were many gospel stories written. That Luke investigated everything so that we can be sure that what he wrote was accurate.

Luke brings these kinds of details into his gospel:

(Luke 3:1-2 NIV) In the fifteenth year of the reign of Tiberius Caesar--when Pontius Pilate was governor of Judea, Herod tetrarch of Galilee, his brother Philip tetrarch of Iturea and Traconitis, and Lysanias tetrarch of Abilene-- {2} during the high priesthood of Annas and Caiaphas, the word of God came to John son of Zechariah in the desert.

Those are facts that wouldn't have been known if the gospel was written a hundred years later by someone who claimed it was Luke.