IS is an operator used to check for specific states of a field, where the value might otherwise be interpreted as an option/text value.
Currently IS is only used for checking for null/empty values (where no selection has been made i.e. when using a multi-select pick list, such as found or fixed in version, or any non-compulsory field) - but in the future it may be extended to support other types of metadata about a field.
Find all incidents with no component assigned:
EntityType = Incident AND Component IS EMPTY
Find all unassigned script assignments:
EntityType = TestScriptAssignment And AssignedTo IS EMPTY
In addition to EMPTY, you can interchangeably use the word "NULL" instead, if this feels more comfortable (i.e. for people familiar with SQL):
EntityType = TestScriptAssignment AND AssignedTo IS NULL