As the previous poster mentioned, most mods allow you to place a regulated mod in locked mode by pressing the + and - buttons together.
If you still have the instruction manual for the mod, you can reference that to determine the button combination to turn off the lock mode.
What Lock mode is used for is to prevent your mod from changing wattage/temperature inadvertantly, but still allows you to vape by pressing the fire button.
I don't find much use in this feature unless I know that I have one of the "beggers" that I am friends with who is bumming a vape off of my mod, let's say for example that I have my airflow closed off on my atomizer to get a more mouth to lung hit, and as a result my wattage is relatively lower than it normally would be for the clouds I typically chuck...in order to prevent the person I hand the mod to from increasing the wattage, and possibly burning out the coils or cotton because he/she didn't realize my airflow was cut off, I would lock it while I am passing it to that person.
Thats just an example of when it might be convenient to utilize the feature, otherwise, I don't use it much at all for anything.