Typically for a failover scenario
isp1 - check ping - distance=5
isp2 distance=10
So all users will go out ISP1 having a shorter distance.
If ping fails the router will use ISP2 as the route of choice, and when ISP1 is availalbe again, users will be redirected to isp1.
The best thread to read up is this one…
http://forum.mikrotik.com/t/advanced-routing-failover-without-scripting/136599/1