The bootloader status specifies the method of flashing Xiaomi phones.

If the bootloader status is unlocked we can install ROM via Fastboot Mode, if locked via EDL Mode (Emergency Download Mode). There are 2 ways to check bootloader status as follows:  

First method - 

• Enable developer options. To do so, follow the steps given below.
• Go to settings → About phone → Tap 5–7 times on MIUI version & it will enable developer options.
• Once the developer options are enabled, follow the steps given below to check bootloader status.
• Go to Settings → Additional settings → Developer options → Mi Unlock Status. 

Second Method -

• Go to Fastboot Mode and connect device to PC.
• Use command "Fastboot devices" to check wether device is connected or not.
• Now use command "fastboot oem device-info" to check bootloader status.

C:\Program Files (x86)\Minimal ADB and Fastboot>Fastboot devices

f4621237d250    fastboot

C:\Program Files (x86)\Minimal ADB and Fastboot>fastboot oem device-info

...

(bootloader)    Device tampered: false

(bootloader)    Device unlocked: false  ---This should be true

(bootloader)    Device critical unlocked: false

(bootloader)    Charger screen enabled: true

(bootloader)    Display panel:

OKAY [  0.009s]

finished. total time: 0.009s