from Merriam Webster
:a balance maintained in an artistic work between opposing forces or elements

I think the above is a good description. there is a nice post here makes reference to it, someone might be able to give you a more indepth explanation, but in the meantime, here are some visual references.

http://www.typography.com/email/2010_03/index_tw.htm

also thought this was excellent
Type study: Typographic hierarchy
http://blog.typekit.com/2011/03/17/type-study-typographic-hierarchy/

another good one to study is

Typography (Emil Ruder, 1967)
http://www.designers-books.com/?p=569

Thanks Mike, so if I understood right, it's the way you design the margins that creates tension? i.e a layout with 4 equal margins has no tension?

Some layouts have more visual tension than others. A layout where everything is centered and symmetrical creates a looks that's very balanced and somewhat formal, but also can be dull.

On the other hand, a layout that has some non-symmetrical elements — rag right type, a large graphic that's not centered on the page, and so forth — has more visual tension and therefore looks less formal and more energetic.

Another way to think about it might be to compare a layout to a garden. Have you ever seen pictures of a very formal garden at some estate where everything is laid out in a big symmetrical grid, with rows of neatly trimmed hedges, everything very formal and symmetrical? Compare that to a beautiful garden that's designed in a very different way, with a big fountain on one side, flowers on the other side, trees placed in seemingly random spots, etc. It almost seem chaotic by comparison, and yet the designer gave it an overall balance that makes it work.

Either type of layout can be appropriate; just depends on the circumstances and what the designer is trying to achieve.

Just to be confusing ;) visual tension can be described as something to be avoided:
http://illustrationclass.com/2007/04/13/avoiding-visual-tension/

I think the general underlying principle is a bit related, but obviously not the same thing. For example, it might maximize this kind of visual tension to set a block of text so that it is printed right up to the edge of the page, but probably that would not be what Muller-Brockmann has in mind.

Thanks for the explanation James, it makes a lot more sense now, I guess it's hard for me to understand it in a visual way, I keep thinking of someone stretching a rubber band and I then try(and fail) to link it to layout, but your comparison to landscape design made it easy to get.

Luma: I do think that too much tension can have a negative effect, personally I like the look but I think it can be very distracting, especially when the block of text is right next to the edge, but I have seen some great designers make it look great, but then again does looking great = good design?

> I keep thinking of someone stretching a rubber band...

Maybe it would help if you thought of it as energy or excitement rather than tension. Compare the energy levels of these two layouts.

The designer needs to create a layout that's appropriate to the occasion. For an ad for a sporting event, a high-energy layout might be very appropriate. But if you tried laying out pages of a novel that way, it obviously wouldn't work.

Tonal contrast arranged asymmetrically in the modernist layout (Tschichoild, c.1930), large areas of light value balanced against smaller darker areas.
This balance creates tension, because it subtle and precarious, and because the centre of gravity is not marked by any graphic element, but is sensed by the viewer.