A signer certificate is the trusted certificate entry that is usually in a truststore file. Deleting a signer certificate from the truststore file will cause users of the truststore file to no long trust the entity represented by the signer certificate.
Read the documentation located in http://www.ibm.com/developerworks/java/jdk/security/iKeymanDocs.zip file for further information.
Result
The Signer Certificates field no longer displays the label of the signer certificate you just deleted.