Free text queries are strings of words, numbers and operators used to search the content of a document. For example, Coveo AND Search finds documents containing the expressions Coveo and Search. Whereas, field queries are used to search the content of a specific field contained in a document or attached to it as metadata. They take the form @fieldname=fieldvalue, where = can be replaced by other relational operators. For example, @sysauthor=John finds documents whose author is John. Furthermore, field and free text queries can be combined to form complex queries. For example, Coveo AND Search AND @sysauthor=John finds documents containing the expressions Coveo and Search whose author is John. Note that field queries are more effective as query refinement tools than free text queries.
See also: What Is Metadata/Meta-Information?, How to Query Date and Time Fields, List of System Fields, What Search Operators Can Be Used in CES?, What Are Fields?