Code locking is used in when you have multiple agents (or the same agent multiple times) running simultaneously, and you are afraid they might interfere with each other by accessing a shared resource at the same time. Since Domino servers are able to run multiple agent threads at the same time either by the agent manager, or in response to http requests, race conditions can occur unpredictably.
The Designer help “Extended examples: lock functions” contains a whole scenario that shows why you might need to do this. The help file describes each of the functions and steps through an example. I don’t feel a need to rewrite it, so if you still have a question about how to do it, could you please be more specific and say at what point of the process you have a problem?