The YPG, the People's Defense Forces, is the armed force of the Syrian Kurds.
It currently controls a quarter of Syria's territory, an area that runs along the border with Turkey.
The power of the Kurds in Syria is seen as a threat by Turkey, which equates the YPG with the terror-branded PKK, the Kurdistan Workers' Party, against which Turkey has fought a protracted armed struggle.
Turkey has been fighting the Kurds in Syria on several occasions since 2016 with the help of Turkish-backed Syrian rebels.
Now Erdogan is threatening a new offensive to push the Kurds back from the border area.
In the video, you see how the YPG became a power factor in Syria - and how Turkey has tried to stop the Kurds' progress.