The Native Emergency Call Handler is used to provide per-device-pool ELINs for emergency calls. When deployed in conjunction with Device Mobility, it can allow for accurate callerID (and presumably the correct associated location information) to be transmitted to PSAP operators for emergency calls.
Think of it as a scaled-down/stripped-down version of Cisco Emergency Responder. It doesn't do notification or provide callback or granular tracking or the other features CER provides. But for organizations that are too small to need CER, but big enough that emergency calls need additional care, this is a decent feature to deploy.
Feature Configuration Guide for CUCM 12.5(1) - Emergency Call Handler
Give this a read and let us know if you have more questions.
Maren