The word mouna has two meanings. The first is silence. Learning to be silent is a practice of pratyahara. Speech, or shabda as it is known in Sanskrit, is an expression of prana shakti or pranic force. A lot of energy is wasted in speech. If you analyze what you say during the day, you will understand that very little speech is constructive or worthwhile. The more you speak the more pranic energy you lose; the less you speak the more pranic energy you conserve. We all like to chat but there is no awareness, no concept, of words being related with prana or energy. If you have to give a lecture for one hour continuously, you feel tired afterwards. Why? You only spoke, you did no physical exercise, yet you feel tired. This is due to the loss of pranic energy. So if you can control your speech, you retain the pranic energy.
Mouna is also important during meal times in order to conserve and direct the pranic energy to fulfil the digestive function. If you can practise absolute mouna when you eat, you will notice that your digestion improves. You can experiment. Try a month of silence, not just one or two days, during meals and observe how well your digestive system functions. Then go back to your regular habit of chatting and see how your system is affected. Mouna is a rule in the ashram. It is very difficult to follow, but those who can benefit from it greatly. For those who can't, the loss is theirs.
The second meaning of mouna is measurement. If you speak measured words then they have power. Mouna also represents a state of mental awareness. Before speaking you remove the chaff which surrounds the words and only say what is required. Then, in the course of time, speech will become powerful.