The Anglo-Saxons lived in warlike times.
The King was a war lord. His job was to provide opportunities for stealing and glory for his followers. If he did not, then he could expect to be murdered and a stronger person take his place.
Below the King, there were freemen. Many freemen were noblemen and owned land and slaves.
Slaves were obtained by capturing the people defeated in battle. Slaves were used as a cheap way of farming the land. If you were poor, you might sell a son or daughter into slavery so that someone else would take over the job of feeding them.