Cover One Man is a pass coverage where one defender has zone responsibility for the deep part of the field and each eligible receiver is covered man to man.
In the screen shot there is one safety deep; he is the player who is responsible for that zone of the field. Each defender in man coverage is indicated with an arrow from him to the offensive player he is covering.
Because there is only one deep safety he will usually be in the middle of the field. The corners will try to force the receivers to the middle of the field where they have help.
There are multiple variations on the coverage, but the general idea of all of the variations is as outlined above.
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