My wife does a bit of photography. She's really good at it, and she really enjoys it. She enjoys it so much that she can get caught up in the moment and forget to pay attention to all of the many details of the shot. There is nothing more frustrating to her than coming in from a shoot, rushing to the computer to take a look at her handy work, finding the "money shot" (as we call it) and discovering that though the lighting, framing and composition were perfect, the subject (somewhere along the way) had gone out of focus.
Out of focus has a very negative ring to it, but it's not always bad when things are out of focus. A lot of times it's a very good thing—when the right thing is in focus.
Depth of Field is the focus range. It determines how much is going to end up being in focus. If that range is large, a lot (maybe all) of the picture will be in focus. If it is small, very little (maybe only one thing) will be in focus. There are many benefits to using the depth of field to your advantage in this manner.
Good photographers can use this to their advantage. They can intentionally cause the ambient images to go out of focus and bring (by contrast) into even greater focus their primary subject. The dangers of not doing this are pretty steep. If you do not focus on one thing (or a few things) you often bring focus to nothing. Your subject gets lost or, worse, conflated with its surroundings.
The same thing happens all the time in life. We can focus on the less important things and miss the truly important ones. We can vehemently argue a false premise to the exclusion of a true one. We can mix and mingle our facts so much that no one can determine where one begins and the other one ends. Focus (and letting things fall out of focus) is essential to life!
It also makes for good design. There are pieces of information in any website, advertisement or newsletter that are more important than others. That's not a bad thing; that's a good thing. Those more important pieces should get the primary focus. They should not be on equal ground with all of the other pieces.
Likewise, there are design elements that should have a more primary focus than their less important counter elements. Good design is able to recognize which one is which (of both the information and the elements) and combine the two appropriately. Important design elements focus on important information—less important elements on less important information, etc.
When you discuss with you designer the work that you need, be very clear about what is important and what is not-so-much. And when ideas are presented, don't focus on the minor things; let them slip out of focus. It looks better that way.