That's an army phrase. It's not used in everyday language. What does it mean? Well, imagine you got a bunch of soldiers lined up in front of you (in long line), and you as the commander stand right in front of them, in the middle, to be seen and heard well enough by everyone in the lineup, and then you order them----- Равняйсь! (everybody turns their head right at the command, except for the one who's at the lineup's right end ) ---- Смирно!!! (everybody returns to the "soldier stance", the head is up and looking straight forward) ------- Равнение на середину! ------- You say that so everyone must then stop being in "soldier stance" and look at you and pay undivided attention to you (remember that you are in the middle).

So that's the general explanation of the command. Of course, there are situations when you say this command before everybody's lined up, like in your own example. But I think I made it clear enough that this command is used by commanders when they have something to say to their lower-ranked personnel.