The European farmer grows grass in summer to give his cattle in winter.But this grass has to be dried be the sun before it can be stored.Hence every hour of sun shine is very important for the former:he has to be quick.So the proverb means to make the best of a chance and not to let a favourable opportunity slip by.
There is a time of everything .If a man loses that moment ,he is put into troubles.:
"Time once past never be recalled."
A blacksmith who never strikes while the iron is  hot cannot make anything.If you have something in hand,wait for the proper time.When the proper time comes,never lose a moment and try to avail yourself of that opportunity.Not to attack when the time is enough causes failure in life.In recent space -research ,"timing"is an important factor.One minute's delay may be fatal to the whole plan.The same thing applies to life in general.
"Life is short and time is fleeting ." 
Every beat of our heart is taking us nearer our grave.All the time we are losing our faculties.If we fail to do our duty during that limited time ,we are doomed .Life ,like the weather is uncertain.Sunshine is the youth of our life.If we fail to win great achievements in our young age,we have waisted it.No man can tell when the sun may go behind the clouds;no man can tell when he may die.Hence the importance of every moment of life.Youth comes but once and opportunities never repeat themselves. 
 
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