Property Tax Portability

At age 65, a homeowner receives a tax freeze for school taxes on a new homestead following such transfer is determined by calculating the percentage of school taxes that are frozen on the current homestead and applying that percentage to the school taxes of the new home. For instance, if the homeowner pays $100 in school taxes for the current homestead, but would have to pay $400 without the tax freeze, the percentage of school taxes paid is 25%. If the school taxes on the new home would normally be $800 in the first year, tax-freeze portability results in school taxes of $200 (or 25% of $800) on that home.
https://www.texasrealestate.com/members/legal-and-ethics/resources/legal-faq/tax-freeze-portability/