Betfair Betting Help?

Was wondering if there is any signs to look out for, to know if the back will increases/decrease or the lay increase or decrease. Also you see the main two ons back and lay, I notice you have two boxes to either side of these, with a an odds figure and value, likewise the two main back and lay ons have the odds value and the the value inside, what are these ons all telling me. Thanks for any help on this.