If you are in the sciences, you need to check PubMed for his publications. Sometimes gaps can be explained by sabbaticals, but generally, having no papers for greater than 2 years is a red flag.
As for what it means, that is very variable. Very long periods are NOT good and you should discount that immediately. So if it's been 3-4 years, I wouldn't even consider it. If it's been 2-3 or less, that is still worrisome. It could be a problem with funding, motivation, or other big issues that require serious consideration on your part. However, know that when people setup new labs, there can also be gaps in publishing so make sure to look at the institutions on his papers to see when he switched from his old lab to this one.