what are Constraints exactly and what should i do? if user try to input data that is not allowed ask him again for data or it is just to let us see the limits of input
Constraint Means the upper and lower limits of Input Data.It also means the Set of Valid Input Data.
1 <= T <= 1000 //Means the value of T in Input Data set is greater than or equal to 1
//and less than or equal to 1000
5 <= A[K] <= 100 //means the value of each member of Array A is greater than or equal to 5
//and less than or equal to 100
Codechef is very serious ,they wont give u the data that is not allowed or that does not satisfy the constraint.Codechef infact guarantee that there is no test data that does not satisfy the given constraint.
" if user try to input data that is not allowed ask him again for data or it is just to let us see the limits of input"
Well it never gives such data .You don’t have to unnecessarily check and skip everytime while Taking Input.
Correct, here is link to FAQ  http://www.codechef.com/wiki/faq#What_should_I_do_with_invalid_input: “There will not be invalid input.”
Constraints are, as you said, the LIMITS (Upper and Lower) of the input data, and are VERY important when considering the solution of a problem. They give an idea of the data types to be used, approach to be adopted etc.
Eg
Lets say input constraint of an integer n is 10^15. (10 raised to power 15)
This means that
1)Using int to store the data will result in overflow (Here it hinted on data type to be used)

If you think of creating an array of this size, you’d exceed memory limit of code and hence arrays cannot be used (here it ruled out any approach using arrays)

With n this large (if its something we have to iterate over), a nested loop or anything using slower algo like O(n^2) will usually give TLE (exceed time limit. Since operations to be performed are a lot)
And many more! You’d get the idea as you progress and practie!
Hope it helped, happy coding!