'Classpath mode' is just how Java has always previously worked prior to the introduction of the Java 9 module system.
It is used in juxtaposition to 'module mode'.
Classpath mode
At compile-time, a Java compiler does not directly
compile a module declaration.
Observability (JLS 7.3) is determined solely by the host system, and
consistently for all compilation units. The host system typically
considers classes on the classpath and/or sourcepath as the entire
observable universe (plus classes named on the command line).
This is how Java works today.
See Project Jigsaw: JDK Modularization Tips