Short answers, pulled from the story.
The Migration Period spans from 300 to 600 AD, though some historians extend the boundaries as early as 300 and as late as 800. Historians traditionally date the start to 375 with Hunnic invasions and end at 568 following Lombard conquests in Italy.
Groups like Goths, Vandals, Huns, Slavs, Franks, Anglo-Saxons, Burgundians, Herulians, Rugians, Scirians, Ostrogoths, Gepids, Avars, Magyars, Bulgars, and Serbs settled former imperial territories during this era. These Germanic peoples moved out of southern Scandinavia and northern Germany after 1000 BC before migrating toward lands between the Elbe and Oder rivers.
Estimates suggest total migration numbers reached between 500,000 and over 1 million people entering Roman lands during these centuries. This mass displacement included Tervingi Goths crossing the Danube into Roman territory in 375 AD fleeing threats from Asia.
Large-scale movements of tribes led to the fall of the Western Roman Empire as central power broke down when barbarian settlements took root throughout former empire lands. Odoacer deposed Romulus Augustulus in 476, and subsequent invasions by Ostrogoths and other groups dismantled political and economic forces holding together the vast domain.
Between AD 500 and 700 Slavic tribes settled central Europe pushing further south east making eastern half predominantly Slavonic speaking. By mid-seventh Serb tribes invaded Albania while ninth saw Haemus Peninsula Epirus become Bulgarian settlements permanently changing demographic picture Balkan region.