JDBC drivers implement the defined interfaces in the JDBC API for interacting with your database server.
For example, using JDBC drivers enable you to open database connections and to interact with it by sending SQL or database commands then receiving results with Java.
The Java.sql package that ships with JDK contains various classes with their behaviours defined and their actual implementaions are done in third-party drivers. Third party vendors implements the java.sql.Driver interface in their database driver.