Sumai

"I invoke Xumae. I will vouch for her, her people... and her dragons, in accordance with the law."

- Xaro Xhoan Daxos

Xumae is a blood oath in the law of the eastern city of Qarth. It allows a resident of the city to vouch for outsiders who wish to enter it. The one who invokes it must cut the palm of a hand and show the wound.

Season 2
Xaro Xhoan Daxos invokes Xumae to vouch for Daenerys Targaryen, her khalasar, and her dragons, to allow them to enter Qarth after the Thirteen refuse them entrance into the city.

In the books
In the A Song of Ice and Fire novels there is no mention of Xumae or any similar blood oath. Daenerys is not refused entrance into Qarth. On the contrary, she's welcomed and well treated because he dragons are considered a marvel