Schema changes refer to modifications made to the structure of a database, which can include adding, deleting, or altering tables, fields, or relationships. These changes are essential for adapting to new requirements, improving performance, or fixing issues within the database. When a schema change occurs, it can impact how data is stored and accessed. Proper planning and testing are crucial to ensure that existing applications and queries continue to function correctly after the changes. Tools and techniques, such as version control and migration scripts, can help manage these updates effectively.