Protect your device by using a screen lock when your device turns on.
Require a numeric PIN or password to decrypt your device each time you power it on.
PROS and CONS
Secure Startup basically encrypts the entire internal storage when Phone is restarted.
Ofcourse no information is left for the hacker attack or in case you lost your mobile before the decryption password is supplied.
It is sometimes required by some security policies (Office 365/Official purpose) but most of the times its not necessary.
The problem is that the device will not be working until the PIN or password is input, no calls, message, internet will work as phone will not come up fully until we enter correct PIN.
Important Note: We need to make sure we should not forget PIN or Password once Secure Startup is configured else phone will not come up after restart.