CSMA/CD stands for carrier sense multiple aceess/collision detection. This is the protocol used in Ethernet network to ensure that only one network at a time is transmitting on any given network.
The CS part of the CSMA/CD stands for carrier sense. What this means is that every Ethernet device has the ability to listen to the network traffic. The Ethernet device will listen to this traffic before it tries to transmit over the network. If the network is clear, the device will transmit the message over the network. If the network is busy with traffic, it will wait it’s turn before sending the message.
The MA part of the CSMA/CD stands for multiple access. What this is implying is that more than one device can have the ability to listen to the network at one time. Multiple devices will listen and wait to transmit their messages.
The CD part of the CSMA/CD stands for collision detection. The CD has the ability to listen to the network and has the ability to detect when the Ethernet devices accidentally transmit at the same time. They will detect when a collision will occur or has occurred.
If the CD detects a collision both of the nodes that are transmitting signals through the network will wait a specified amount of time and then each one will independently try to send the data through the network again. Kind of like when you are walking in a narrow hallway and there is someone in front of you walking the other direction. You both pause and try to step to one side or another or wait for the other person to pass before trying to continue down the hallway.
In short, the CSMA/CD is like a traffic cop. It listens for the traffic going both ways and tries to ensure that there are no accidents. If there is going to be an accident or is an accident the CD (traffic cop) notices and then directs traffic.