Can anyone explain me in a layman language what is saade sathi ..transit of satu!

Can anyone explain me in a layman language what is saade sathi ..transit of saturn and how it affects in detai

Saadesathi means the transit of saturn over the three houses,viz. the12th house/sign from Moon,the house/sign in which the moon is located and the second house/sign from the Moon.As you would be aware it takes saturn two and half years to pass over a house.Thus for three houses seven and half years and hence the name Sade Sati.
Now Saturn is a dark planet.Unlike other planets which throw light,saturn causes darkness.Further,moon is the only heavenly body amongst the ones considered in astrology which has the capcity to create some decent amount of light in darkness.So when a dark planet like saturn transits over the moon the only body capable of creating some light in darkness,there is more darkness.Now you may ask what darkness means.See moon is considered to be mind in astrology and thus the effect of sade sati is primarily on the mind.There may be depression,dark thoughts,thoughts about unforeseen fears etc.If however the dasha and bhukti as also the transit of jupiter during sade sati is good then the bad effects may be limited to these only and the person may get everything else.If however these are also bad then some negative events too could occur during this period.