The Kings, who are the rulers of their kingdoms, need funds to administer his kingdom. In Vedic era the kings used to collect cess at the rate of 1/6 of the earnings of the people in their countries, and used to protect the people like their children.
Sage Valmiki in his Srimad Ramayana, which contains information about the way life in that era, describes this issue:
अधार्मः सुमहान् नाथ भवेत् तस्य तु भूपतेः | यो हरेत् बलि षड् भागम् न च रक्षति पुत्रवत् || (Aranya Kanda 6th Sarga 11th Sloka)
"Oh! , very great a-dharma, wrongness, will occur to that king who takes the sixth part of the country's produce as cess, but does not safeguard his subjects like his own sons. "
Then what about the Sages, who reside in the Forest, practising ascesis, will not pay any tax to the King, as their earnings are practically Zero, except providing occasionally any advice to the king. However, the king used to protect those ascetics also.
Does it mean that they did not pay any tax to the kings? The answer to this question lies hidden in Srimad Ramayana itself.
1/4 of ascetic power of the ascetics will accrue automatically to the Kings, who righteously protects his subjects.
यत् करोति परम् धर्मम् मुनिः मूल फल अशनः | तत्र राज्ञः चतुर् भागः प्रजा धर्मेण रक्षतः || (Aranya Kanda 6th Sarga 14th Sloka)
"He,who righteously protects his subjects, will get one fourth of the merit of great duty-bound deeds performed by each individual sage, say yajna-s, ascesis, meditation etc. performances. "
That is the tax the Sages used to pay to the King.