Stick to the topic on hand. If your article is about the benefits of an item you just bought, then do your best to strictly write about the benefits of that item. If you write about the top reasons to visit a country you have been to, stick to the reasons and don't go off giving a history lesson about the country, unless of course that's one of the reasons you think your readers should go and visit the location.

Your Readers Know What They Came For

Remember, when your readers arrive at your article, they are going to read it for a reason. If you write an article about why you should visit a specific destination, that is what they are going to want to read about, if that makes sense?

Example

Let's say I write an article about The Benefits Of Giant Sets. I am not going to trail off the topic and discuss what giant sets are and how to do them. I'm not going to write about who created giant sets or anything like that.

I'll simply stick to the benefits of giant sets. If you trail off of topic, then you'll easily become distracted and will lose your flow. Always stick to the topic at hand.