Generally we say, the higher the cited by number is, the more impactful the research is. It means that, in this case, over 3300 articles referenced this particular paper. Considering it was published in 1998, that's a good amount of cited bys as it has had time to be read and thoroughly studied.
But what if I told you that this article was retracted in 2010 for fraudulent research? And that even though it was retracted, it has already been cited over 3300 times and is now part of public discourse. That this particular scholarly article was based on shoddy research and literally started the anti-vax movement?
Who you cite matters! We have a responsibility to thoroughly evaluate our sources.
You might need to find more information about the study/studies being described, in order to assess the article's accuracy or fill in the gaps. If the article doesn't answer some of these questions, chances are it's not telling the whole story.
Does the article support its claims with scientific research?
Did the study actually assess what's in the headline?
Was the research in humans?
How many people did it include?
Did the study have a control group?
Who paid for and conducted the study?