Focus (also referred to as "dominance" or "emphasis") means that the objects on the page do not compete with each other for your attention (like a stamp album), there will be one item that has more emphasis than the others thus ensuring clarity & effectiveness of communication.
A typical stamp album places equal emphasis on all items. The viewer doesn't know where to look for the information; nothing stands out except for the top-left to bottom-right eye-flow convention.
Generally, avoid the stamp-album metaphor in your layouts.
(This is, of course, a design compromise; a stamp album needs to hold lots of stamps; it doesn't need to adhere to design fundamentals, it's chief purpose is NOT to communicate or advertise).
Here's a great example of "focus":
The product shot of the beer bottle places emphasis on it by virtue of its central position in the shot as well as the bottle's image clarity relative to the surroundings. This is referred to as a "hero shot". It's called a hero shot because early pulp-fiction would often portray their story's hero/heroine on the front cover like a figurehead for the publication; pointing to the sky or posing with a monumental backdrop. In the photo the hero is the bottle.
Focus and Information Hierarchy
The goal of any designer is to make the layout "easy to read" or "easy to understand". This is vitally important if you're designing a billboard on the side of a highway with readers driving past at high speed.
The expression "information hieraRchy" describes how the elements on a page have levels of focus or emphasis according to the following:
The object's size
The object's position relative to the page or frame or "framal reference" aka "ground"
The object's position relative to other objects on the page
The object's colour
The "nature" of the object; is it type? Is it a photo? Is it engaging artwork? Is it a model wearing a neon-orange bowler hat while hanging from a cliff?
The amount of "white-space" on the page relative to the object.