Lets say you want to buy rio for $60 a share
it closed last night at $55 a share
This morning it's going to open at $59 each because lots of people want it (you don't know its going to open at this price though until the market actually opens.. you can get some idea from the VWAP but there are lots of changes in the last few seconds)
Because your limit was at $60 you will be filled on the open price
If your limit was at $56 then you wouldn't get filled, becuase the price gapped over your buy limit.
You don't actually pay $60 for the share, it's just what you are saying is your MAXIMUM buy price. You will pay the market price at the time of buying.
Hope this helps
Brad