What is EFS and MSL in android? I just bought a flashing box for Samsung mobiles and I am trying to use efs and msl imei repair options in Z3X pro box. I just want to know about them let's say how they works and how to write certificates using this.
Cert file is used to unlock the network so that another sim card can be used with the phone or changing the IMEI number, country code... (all these are illegal in many countries).

The EFS is a special partition dedicated for your phone's info like radio signal info, imei, serial number and lots of files revolving around your SIM card and Wifi, Bluetooth (this includes your MAC address for all the radios of your phone).  Imagine the EFS as a big folder containing all of the important stuff that makes the "phone" part of your phone.

The MSL just allows you access to dangerous menus that could actually screw your device up if you set something wrong. So, in your case, Z3X needs to bypass MSL to be able to modify any setting, such as have write access to the EFS partition and change IMEI/unlock the network, etc.

There are thounds of videos out there on how to use that feature in Z3X Box.
You can find more help from here.