This setting will allow only the ops at or above the level you set to change your topic. Anyone who is not authorized to change the topic who tries to change it will have the topic reset back to what it was. The only time when someone of lower status than you set the topiclock to can change the topic is if no topic has been set in the room. As soon as there is a topic for services to set it back to, the topiclock will be enforced. Services will also set the topic back after the room empties when someone rejoins. Topiclock in any position other than off includes the functionality of keeptopic, and if set on, keeptopic will automatically be removed if it was on.
Example:
/msg chanserv set #support topiclock aop
Topiclock is a good way to control what topic is showing for your room. Since the topic is one of the first things people see about your room, you want it to represent you well. What level you want to restrict topic changes to will depend upon how you choose to give people AOPs and such, and how important the topic is to your room. If your room is a place just for you to hang out with your friends you might prefer to leave it unlocked and watch as people set amusing topics and change it often.