forName is a method in class
java.lang.Class. You can look it up in the API documentation.
What it does is find a class by name and load the class. Part of loading the class is running the static initializers in the class. Normally,
JDBC driver classes internally have a static initializer that registers the driver in an internal JDBC driver registry, so that JDBC knows the driver class.
Note that in the latest version of the JDBC API (I think JDBC 4.0 if I remember correctly, which is included with
Java SE 6) the mechanism to find and load drivers has been changed, so that it isn't really necessary anymore to do Class.forName("...") to load the driver. You only need to do this if you're using an older driver or an older version of Java.