Elsevier Computers and Security "With Editor" status just after 12 hours - what does it mean?
In electronic submission systems the manuscript (MS) is handled according to a certain work flow. When a MS is submitted it has to be assigned to an editor who will check, for example, if the MS is appropriate for the journal or adheres to journal "Instructions for Authors" (or equivalent). The editor will then either start to assign reviewers to your MS or pass it on to an Associate editor (equiv.) to do the same. Then several rounds follow that is beyond this post. Thus there are lots of steps that your MS will undergo and the system will signals some subset of all steps involved. 

So the likely interpretation of your observed change in status is that the journal, possibly through some administrator, has assigned the MS to an editor for further processing. This does not necessarily mean much but it shows your MS is on its way forward in the journal work flow. The process of moving a MS to a new step can be done very quickly and will largely depend on the timing of the person doing the chore. In the journal where I am editor, there is a six day limit to move MS from submitted to an editor. Usually the move is made within 24 hours and shorter depending on when the MS is submitted and when the admin checks the newly submitted MS list.

Se also this Elsevier link