A license is a legal document, usually issued by a government agency, which gives an individual the right to be compensated for offering their services. Countries and jurisdictions have different (or no) laws governing the professional practice of reflexology. Some require practitioners to have a government-issued license in a related discipline before they can legally accept any compensation for giving a reflexology session.
Before you book a reflexology session with someone advertising the service, you would be wise to Inquire about the practitioner’s training in reflexology and whether they are duly licensed and allowed to offer their services for compensation. For United State residents, you may wish to start with reading this document.