A signatory is someone who signs their name on a document or agreement. This person could be one of many people putting their names to something.
You might hear about signatories when reading about important international agreements like treaties or contracts. These are often signed by leaders of countries, and sometimes by special ambassadors or representatives too. Think about big events like the Paris Climate Agreement - lots of nations had leaders who were signatory to this deal. Sometimes you'll see a list of all the signatories together, so it's clear who was involved in signing that particular document.
One who signs or has signed something.
Example: John Hancock is famous for being the first signatory to the American Declaration of Independence, and for writing his name large.
