Ok I will try to expalin this with an example and maybe the best example there currently is. Say you have a mage and you want to sheep target X and kill target Y. If you make a focus macro for sheep, it would work something like this....

Target X and set focus and sheep, now target Y and blast away, wait X broke sheep, no problem just hit sheep without ever switching from Y.

Its basically telling the macro to remember X and when you sheep X, it will sheep X without having to be the current target. There is an immense guide on macro creation made by a guy named Cogwheel on the wowwiki or WoW forums. Maybe I am not the best to explain but its the general idea.