To use all the neat features from the microG project, which allows you to use all features of your Android smartphone without those shitty, proprietary battery-consuming Google blobs, your system is required to support signature spoofing. Currently only very few custom ROMs have built-in support for this feature, luckily you can use Xposed or a patching tool to add the feature to the systems that don’t have it.
But: What is all this about? Is signature spoofing a problem when not using microG? Will it influence my security?