What does “Failure [INSTALL_FAILED_OLDER_SDK]” mean in Android Studio?

After I changed

defaultConfig {
    applicationId "com.example.bocheng.myapplication"
    minSdkVersion 15
    targetSdkVersion 'L' #change this to 19
    versionCode 1
    versionName "1.0"
}

in build.gradle file.

it works