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.
Normally there are several reviews that an editor will request and then gather so that a decision can be made. The decision itself will take some time. Most likely, the editor asked for a review, setting a deadline, and the reviewer didn't meet the deadline and so this is the response. The deadline set by the editor would be some days, at least, before the expected end of the review period.
On the other hand, stating that review takes a month is, for most journals, just a guess rather than a hard limit.
In effect, it is an internal thing, not something you need to take action on.