A natural key is one constructed of data that already exists in the table. For example using latitude and longitude in a table of addresses. Or the social security number in a table of employees. (Before you say anything, yes, the social security number is a horrible primary key. Be patient.)
My personal preference is to use surrogate keys most of the time and put unique constraints (or unique indexes) on the natural key. There are some occasions in which I’d deviate, but ceteris paribus I’d pick this strategy..