The dot (.) operator is used in Spark SQL to navigate through the nested structure of a JSON object. For example, if you have a JSON string in a column named json_data with content like {"name": "John Doe", "age": 30, "address": {"city": "New York", "country": "USA"}}, you can access the "city" field within the "address" subfield by using the expression SELECT json_data.address.city FROM your_table_name;. This will return "New York". The dot operator allows for chaining to access deeper levels within the JSON.