A more thorough explanation:

de tallagio non concedendo

de tallagio non concedendo is a law that says the king or his heirs cannot impose taxes without the agreement of important people in the country like archbishops, bishops, earls, barons, knights, and other freemen of the realm. This law has been used to support the idea that only Parliament can impose taxes.

For example, if the king wanted to impose a new tax on the people, he would have to get the agreement of the archbishops, bishops, earls, barons, knights, and other freemen of the realm before he could do it. This law helped to limit the power of the king and give more power to the people.