what does this mean and how do you know

Here's how I do it.  I usually divide the water column into three sections.  If the Lowrance shows fish in either the top or bottom third....they're a whole lot more catchable than if they're located in the middle section.  I've been out on days when literally ALL of the fish seemed to be hanging in that miserable middle portion > .  In that case, it's "Houston we have a problem"  ;D  , cuz those are a huge challenge to get to bite.

thats a good way to look at it :