The main concept is that only one packet may be transmitted at a time. Then sender stop sending the next packet and wait for the acknowledgement from the reciever. Once sender receives the acknowledgement from the reciever it sends the next packet.
The working of a stop and wait protocol may be explained as-
Sender sends a data packet to the receiver.
Sender stops and waits for the acknowledgement for the sent packet from the receiver.
Receiver receives and processes the data packet.
Receiver sends an acknowledgement to the sender.
After receiving the acknowledgement, sender sends the next data packet to the receiver.