The communication between sender and receiver occurs in both directions in half duplex transmission, but only one at a time. For example, in walkie-talkies, the speakers at both ends can speak, but they have to speak one by one. They cannot speak simultaneously.
In full duplex transmission mode, the communication between sender and receiver can occur simultaneously. For example, in a telephone conversation, two people communicate, and both are free to speak and listen at the same time.