A license is a legal document, usually issued by a government agency, which gives you the right to be compensated for offering your services. Countries and jurisdictions have different (or no) laws governing the professional practice of reflexology. Some require you to have a government-issued license in a related discipline before you can legally accept any compensation for giving a reflexology session.
It is in your best interest to familiarize yourself with the requirements and legalities of the country and jurisdiction (state, province, county, city) in which you plan to practice. For United State residents, you may wish to start with reading this document.