Imagine you want to talk to your friend who is sitting far away from you. But, you can't shout loud enough for them to hear your voice. So, what do you do? You use a telephone or a mobile phone to talk to them.
Now, when you use a phone, your voice gets converted into tiny electrical signals and is sent through wires or wireless signals to your friend's phone. Similarly, a modem is a device that helps to send computer data through telephone lines or cable wires.
Command mode is like telling the modem what to do. It's like telling your friend to hold on for a moment before you start talking to them. In command mode, you have to give instructions to the modem on how to send and receive data.
Data mode is when the modem actually sends the data from your computer to another computer. It's like talking to your friend on the phone. In data mode, the modem receives and sends the data that you want to send.
In a nutshell, command mode is like giving instructions to the modem, and data mode is when the modem actually sends and receives the data from your computer.