Hi !!
The monitor icon on track for ear what we record, when we should use it ?
Is it only when you are playing a instrument?
Is it recommend to put a earing back on the headphone of the singer for he or she ear
is self voice combined with the ‘‘beat’’ ?
Enable the “Monitor” function every time you are on any type of track (audio or midi) recording vocals or instruments. Typically the sound engineer (or whoever is running Cubase) needs to hear the track and what is being recorded. Basically it is the same for the talent who sing and play instruments. Surely they will be wearing a headphone (or some other in-ear device) to hear the click (beat) and whatever tracks that may have been previously recorded or are being recorded at the same time they are recording something. If they can’t hear themselves and what they are singing or playing in the mix it will be very difficult for them to keep the beat and create whatever expressions they want their performance to have.
Remember to turn off the “Monitor” function after recording to hear what you recorded on audio tracks.
Regards 
Hey, tank you a lot for you fast answer! I have no more question about that. 
There are other ways to monitor incoming audio, depending on whether you are using the sound card mixer (e.g. RME Totalmix or MOTU Cuemix) or Direct Monitor.
Also check out the automatic functions in Cubase Preferences for Tape Style or Manual monitoring.
This is quite a complex subject, many options available, Cubase has most of them covered.