If there is a colleague locking something and you need a quick way to draw his/her attention, you can "send a message" from the system. Check who's online and what's happening on your or the other application servers.
Task Handler functionality is normally something you shouldn't have to deal with in your applications. There (should be) no business application logic that would require throwing a popup message into the end-user's face, which pops up immediately and which shows a big red cross. Don't build this into your applications, or don't be surprised if your user community will ask you to take it out again.
Function module TH_POPUP can be called from the SE37 test bench. Simply fill in the checkbox "Upper case / lower cased" at the top (unless you want the message to be upper cased), supply the client and receiving user-ID along with the message.
Note that the "Task handler" suite of function modules is quite elaborate: there's almost 100 TH_* functions in function group THFB. Among which:
TH_SERVER_LIST- returns the list of active servers on the systemTH_WPINFO- retuns the list of work processed on a serverTH_USER_LIST- returns a list of logged on users on the systemTH_SAPREL- Version information for the system, quite elaborate..