JAR files contain classes, perhaps something like this.

com/example/foo.class
com/example/bar.class

Let's say the JAR file is located at 

/usr/local/jars/example.jar

In this scenario, if a Java application wants to use foo.class and bar.class in example.jar, the Java application will need to include /usr/local/jars in it's classpath. For example, if you have a Java application that needs to use foo.class or bar.class in example.jar, the -classpath option can be used to include the .jar files in a directory, like this.

/usr/bin/java -classpath /usr/local/jars myapp

On Linux, this command will temporarily update the CLASSPATH variable to contain a certain directory.

CLASSPATH=/opt/jars

The echo command can be used to verify that CLASSPATH now contains /opt/jars.

echo $CLASSPATH