What is the meaning of "review is overdue"? I have a status for my paper " a review is overdue"  after 20 days of reviewing my paper, however it is mentioned in the web site that review takes 1 month. What does this means?
This means a review is overdue: the editor invited a reviewer with a given deadline, which has passed. It seems the editor's deadline was only ~20 days. That's pretty fast, but since the website is claiming that review takes one month, it might be necessary.

The good news is the journal treats its authors well; many journals simply report your paper is "under review" and leave it at that until the decision is made. The bad news is, if the reviewer doesn't submit a review, your paper might take another month (or more) to process if the editor elects to invite more reviewers.

In any case there's nothing to do except wait.