Er Wang San Ke

Erwang Sanke (二王三恪) or "the two crownings and the three respects" is something Chinese emperors did to honor the previous dynasty. For example the Sui dynasty came directly before the Tang. So the Tang may honor descendants of the Sui royal family. The Tang would do this by giving these Sui descendants land to rule over. The Tang said you can control these lands and use laws of the previous Sui dynasty if you wish. There may also be a sacrifice at a religious place.
This was done to show respect to previous dynasties. This was also to make a peaceful transition of power.
It has been done in every major Chinese dynasty from the Xia to the Qing.
Sometimes people would pretend to be the descendants of the previous dynasty just to get this benefit.
Name.
Er wang 二王 means two rulers. This refers to the current ruler and the previous ruler.
San ke 三恪 means three respects. This could refer to how this was done for the first three generation descendants of the previous ruler (their children, grandchildren, and great grandchildren).