The central panel has many of the features seen in woodcuts of this saint. On the left stands a walking staff with a pilgrim's hat and scrip. These refer to Bridget's pilgrimages to Compostela, Jerusalem, and Rome. As seen in the detail at left, the hat has a "veronica" badge, an image of the face of Jesus purported to be imprinted on Veronica's veil.
In a cloud above and to the right of the hat, God the Father cradles the naked body of the Son, as in Throne of Mercy and Death of Jesus images. This is mirrored by the image on the right of the Virgin Mary cradling the naked Christ Child. Her golden crown with three trefoils is seen in many Brigittine images of this era.
Between these two figures and directly above Bridget's halo is the ihs monogram that represents the name of Jesus. Above that the Holy Spirit hovers in the form of a dove.
The two shields on the bottom left of the central panel are common in images of St. Bridget in the late middle ages. The one with the rampant lion appears to be a family device; the SPQR in the other one refers to her pilgrimage to Rome. (SPQR, for Senatus populusque Romanus, appeared on Roman coins of the 15th and 16th centuries.)
Finally, the inscriptions. The scroll bearing the nuns' prayer says in a mix of Latin and German, O birgitta sponsa ieshu christi Bitt got für uns, "Oh Bridget, spouse of Jesus Christ, pray to God for us." The monks are praying, O pater de celis miserere nobis, "Oh Father in Heaven, have mercy on us." This phrase appears in various medieval liturgies, e.g. Long 447 (Easter Vigil), Huntington Library Books of Hours HM48 (Trinity Sunday) and 1101 (Suffrage of the Trinity), and many litanies into the modern age. The legend beneath the shields and crown reads O her ihesu christe [illegible word: something like einzig?] son des almächtigen gots erbarm dich ober uns, "Oh Lord Jesus Christ, [only?] son of Almighty God, have mercy on us."
Read more about images of St. Bridget of Sweden.