Recovery, in Android, refers to the bootable partition, in which the recovery console is installed.
It contains tools to help repair your installations as well as install official OS updates by using a combination of key presses or instructions from a command line sometimes.
Android is open, meaning the recovery source code is available, so that building a customized ROM is relatively easy.
1. Recovery mode process begins from turning off your device.
2. Wait few seconds press and hold combination of these buttons: power + home + volume down.
3. After a moment of holding there should appear LG logo on your screen, you can let go off the buttons then.
4. Now your phone will boot Recovery Mode screen.
5. This is it! Now you know how to put your LG Nexus 5 into Recovery Mode