It is explained in a simple way on wiki,
The JDBC type 1 driver, also known as the JDBC-ODBC bridge, is a database driver implementation that employs the ODBC driver to connect to the database. The driver converts JDBC method calls into ODBC function calls.
The driver is platform-dependent as it makes use of ODBC, which in turn depends on native libraries of the underlying operating system the JVM is running upon.
Hope the image below helps as well.