It's a person who is contractually bound to what is essentially slavery with pay.
So, for example, you want to move out of your rat hole of a country but cannot afford to do so. A person in the country you want to live in says they will sponsor your move, but in turn you must work in their factory for 10 years while making a wage you can only barely get by on. This person may even require you to hand them your Visa so you cannot freely leave.
As the years go by, you cannot afford to live on the wage you're being paid, but no worry because if you agree to remain under the contract another couple years they'll help you... and then you'll need to agree to a couple more... and so on. You have essentially become a slave, all while believing you were pursing a better life.