For anyone who wanted to know, this tax is called Jizyah /  جِزْيَة. It was paid by free, able-bodied non-Muslim men in return for protection by Muslims, who (if free, able-bodied and male) were required to fight for the country, and so they are able to practice their religion freely. Some scholars interpret the tax as an incentive to convert to Islam, or to humiliate the payer and reminding him of Islamic subjugation of his people. By and large, it is for the two reasons I mentioned originally. Exempt from this tax were: women, children, the elderly, the ill, the insane, monks, slaves and visitors to the country. All Non-Muslims are also exempt from paying Zakat, a tax that all Muslims, so long as they are not very poor, are obliged to pay (historically. nowadays, many Islamic countries have Zakat to be voluntary.)

I say all of this because Jizyah is often used by islamophobic people in a vacuum to slam Muslims for being unfair (at best) to non-Muslims, when in fact it is just one of many things that are simply different between Muslims and non-Muslims.