JAR (Java Archive) file is a package file format that contains many classes, metadata, and resources like image, audio etc., that is necessary for a Java application. A JAR file is a Java application compressed in a single file which contains all the resources required to run a Java application.
The command mentioned in the question is used to run a JAR file. When you have written a Java program, the command to execute it is:
java <name of the java program file>
Similarly, when you want to execute a Java application that is present as a JAR file, you use the below command:
java -jar <some jar file>.jar
Here, you have to replace <some jar file> with the name of the JAR file. Suppose you want to execute Example.jar, then the command would be:
java -jar Example.jar