As far as I understand,Confidence Interval of a parameter is an interval which contains the true value of the parameter at a certain level of confidence. But, why do we need it?
Confidence intervals express the uncertainty in your estimate of the parameter. The confidence level depends on how frequently you would want in repeated sampling the interval to contain the true value of the parameter. So 99% expresses greater confidence than 95% but it also leads to a wider interval. The choice of confidence level is up to the user and depends on the application.