Advanced Searching - Fields Reference — A field in JQL is a word that represents a JIRA field (or a custom field that has already been defined in JIRA). In a clause, a field is followed by an , which in turn is followed by one or more values (or functions). The operator compares the value of the field with one or more values or functions on the right, such that only true results are retrieved by the clause.
Advanced Searching - Operators Reference — An operator in JQL is one or more symbols or words which compares the value of a on its left with one or more values (or functions) on its right, such that only true results are retrieved by the clause. Some operators may use the keyword.