massless rope that attaches crates , masses and blocks In exercices that involves crates , sliding , ropes and pulleys ,  they often say "masseless" string /rope , why ? what does it physically mean ?
 A: If you have two blocks connected by a string, you actually have three objects.
In a careful analysis you have to apply Newton's Laws to each one of the three objects to arrive at a dynamical equation for each object.  You also have to find constraint equations that couple the dynamical equations.  
I recommend that you do that analysis.  Then with that solution in hand, let the mass of the string go to zero.  You will see how the solution is different, and you will also see that the solution is simpler.  It turns out that in the massless string case the magnitude of tension on one end of the string is equal to the tension on the other end.  This is not true in the case of a string with mass.   By taking the string massless from the start, you can use the result that the tensions on either end of the string are the same, and thus eliminate the string from the analysis.  It's simpler.
So for introductory problems, we choose the mathematically simpler massless string.  We use (often with no proof at all) the result that the tension is the same on either end of the string.  But this obscures some interesting points that will have to be explored later.
Practically speaking, we can imagine a very fine string and very heavy objects.  The massless string analysis provides a good approximation to that case.
A: That is, you don't have to add the masses of ropes and strings along with the load they are connected to.  Their mass are very low as compared to mass itself, and would not affect much in weight and other physical quantities that are mass dependent. 
