what was the important lesson siddhartha learns ? And how do you apply it to your life ?

I think the letting go of desire as well as the ability to be passive and listen are important lessons.  Our world is filled with desire. Material wealth, sex, ego....are just a few examples. Siddhartha has to let these go to experience truth. Siddhartha must learn to be passive and not impose his will in order to do this. This is a good lesson for us in this world full of distractions and constant validation.