A Tributary state is a state that is subordinate to a more powerful neighbour. It was a form of subordination in pre-modern times.
The tributary sent a regular token of submission (tribute) to the superior power. This token often took the form of a substantial transfer of wealth, such as the delivery of gold, produce, or slaves, so that tribute might best be seen as the payment of protection money. But there were other ways to show submission as well. A perfect example of a state like this is the Himalayan territory of Sikkim.