How does https become a secure protocol?

https does not 'become' a secure protocol, it 'is' a secure protocol. however, it is 'common practice to intially use the http protocol and 'switch to' or 'use' the https protocol for 'secure' or 'encrypted' communication. information exchanged using the http protocol is exchanged in the 'clear' or using 'plain' text while information exchanged using the https protocol is 'encrypted' (using any number of encryption and or signing mechanisms) prior to exchange. https is an industry agreed upon standard that references the 'secure exchange' of http information.