What is the difference between focus and concentration in the following context?
High-flow activities require focus and concentration; your mind is actively engaged in what you are doing.
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Technically, focus relates to the (better definition of) lateral extent (breadth/ range/ coverage), while concentration is about depth.
In other words, as you focus more and more, your attention is confined to a smaller and smaller area. When you concentrate on something, the depth of your attention is greater.
In practical terms, it is possible to visualize the difference in some instances, while in others it may be merely a matter of opinion.
focus (http://oxforddictionaries.com/definition/english/focus) 1 the centre of interest or activity
However, dictionaries also define focus somewhat synonymous with concentrate, at least in literary use.
Quite possibly someone will provide an answer attempting to show how focus and concentration somehow means something more than either word on its own.
So far as I'm concerned, in OP's specific context it's just tautological repetition for the sake of emphasis. This is perfectly normal in English, and in no way, shape, or form is it a "defect".
In this context, as @FumbleFingers says, focus and concentration mean pretty much the same thing.
In slightly different contexts, however, focus (as a noun) may also be used to designate the object of the action. This is infrequently encountered with concentration, except in the academic area of concentration.
Concentration, on the other hand, is somewhat more likely to be used when the writer wants to emphasize the action itself, or the effort expended.
And wider afield, of course, usage is still more distinct: you are very unlikely to hear of a lens bringing light rays to a concentration, or of enemies of the state being imprisoned in a focus camp!
Within the context of mental activity:
Focus: Allocate mental resources to a particular task -- e.g. to read this line or that paragraph or that book
(Pure) Concentration : Allocation of mental resources to a particular task over the duration of (uninterrupted) time that is required to execute the task.
Focus is like the smallest unit of Concentration. Concentration occurs when contiguous units of focus are allocated to execute the task.
The main difference : Concentration is required to execute the task, focus is require to direct attention to the task
Not staying focus, simply means the lack of concentration