Mā malakat aymānukum ("what your right hands possess", Arabic: ما ملكت أيمانکم‎‎) is an expression referring to slaves.

Narrated 'Abdul 'Aziz: Anas said, 'When Allah's Apostle invaded Khaibar, we offered the Fajr prayer there yearly in the morning) when it was still dark. The Prophet rode and Abu Talha rode too and I was riding behind Abu Talha. The Prophet passed through the lane of Khaibar quickly and my knee was touching the thigh of the Prophet . He uncovered his thigh and I saw the whiteness of the thigh of the Prophet. When he entered the town, he said, 'Allahu Akbar! Khaibar is ruined. Whenever we approach near a (hostile) nation (to fight) then evil will be the morning of those who have been warned.' He repeated this thrice. The people came out for their jobs and some of them said, 'Muhammad (has come).' (Some of our companions added, "With his army.") We conquered Khaibar, took the captives, and the booty was collected. Dihya came and said, 'O Allah's Prophet! Give me a slave girl from the captives.' The Prophet said, 'Go and take any slave girl.' He took Safiya bint Huyai. A man came to the Prophet and said, 'O Allah's Apostles! You gave Safiya bint Huyai to Dihya and she is the chief mistress of the tribes of Quraiza and An-Nadir and she befits none but you.' So the Prophet said, 'Bring him along with her.' So Dihya came with her and when the Prophet saw her, he said to Dihya, 'Take any slave girl other than her from the captives.' Anas added: The Prophet then manumitted her and married her..." Sahih Bukhari 1:8:367

So yeah, if you want to be super technical they start off as captives and then become slaves.