By listening to what other people are saying you can react in the correct way - you can agree or disagree.
To be a good communicator, you need not only to be able to speak clearly but you need to be able to listen.
Listening by yourself
If a teacher, parent or friend is explaining something to you on your own, it's polite to nod when you understand or say something like, 'I see'.
This shows the person doing the expkaining that you undserstand and they can carry on to the next point happy that you are understanding what they're saying.
If you don't understand you can use your face to look puzzled or just ask if they can explain it again.
If you just sit and listen with no expression on your face, the person doing the talking has no way of telling if you know what they're talking about - infact they don't know whether you fully understand, don't understand at all or if you don't care either way!
Listening in a group
If you're in a group (perhaps a classroom) it's not always possible to let the teacher know that you understand verbally. In these circumstances you can use your face. If you have a good teacher she will be continually looking at her class to see if people look confused.
Listening to a debate or persuasive speech
If you're listening to a debate or a persuasive speech where you'll have a chance to ask questions and agree or disagree with the speaker, it' a good idea to take notes of any points you strongly disagree with.
Try not to pick out silly little points - stay with the main ideas of the debate and make sure you're listening carefully to what is being said so that you can argue convincingly.