This is a complex question, which falls under the purview of Congenital Cardiology, meaning abnormal vascular or cardiac formation during in utero development. An extra vessel from the SVC (Superior Vena Cava, which drains blood from your arms and head back to your heart) may represent an anomalous extra venous drainage back to the heart. Without more detail it is hard to determine what the structure is exactly. All the questions you ask here are very appropriate for the specialist you will see shortly.
Without seeing the images of the testing, and the exact report (which are not appropriate to view in this setting anyway, but again will and should be reviewed with the specialist you will see), it is hard to answer much. However, if it is an anomalous vein, that usually is something that can be monitored over time, but should not have an adverse effect on your daughter's mental or growth development. Often, these are completely benign. However, sometimes they may be associated with more significant anomalies, and that should be discussed at the visit. Your specialist will be able to know what concerns, if any, should be had, based on the anomaly found on the testing you've had thus far.