Signature is that the developer encrypts the apk file to prevent the program from being pirated. Because the Android system is a linux kernel and programs are distinguished by package names, others may develop an application with the same package name. At this time, the security desk system will judge that if the signatures are the same, it will be replaced. If the signatures are different, it is not allowed to install applications with duplicate package names. As for when you say to change it, usually developers can't change their signatures. If you change the signing program, you will not be able to update the installation.