In a test there will be some action if any action in a test 
is useful in another test then we are going to use that 
action as re-usable action then we going to that action in 
another test

NOTE :We can use a action of one test in another test if 
that action is change into re-usable action
 
if any wrong in this answer inform to me plz.........

these r the actions which r reusable in another actions. we 
can also make non reusable actions into reusable actions .
 for this we have 2 follow this navigation
record our reqs operation->step menu->action properties-
>select reusable actions

Reusable actions are like functions in programming. when 
you have a process that needs to be included in several 
places in the testscript you can record and save it as a 
reusable action. (Check the reusable checkbox while 
recording the action). Then you can call this action from 
other tests instead of recording and modifying each time,