Where is the Keytool application?

keytool is a tool to manage (public/private) security keys and certificates and store them in a Java KeyStore file (stored_file_name.jks).
It is provided with any standard JDK/JRE distributions.
You can find it under the following folder %JAVA_HOME%\bin.

keytool is part of the standard java distribution.

In a windows 64-bit machine, you would normally find the jdk at

C:\Program Files\Java\jdk1.8.0_121\bin

It is used for managing keys and certificates you can sign things with, in your case, probably a jar file.

If you provide more details of what you need to do, we could probably give you a more specific answer.