Locking a thread prevents members from posting in it any further. As long as a lock is in place, only staff with the power to respond to locked threads will be able to post in that thread.
When creating a thread, you can automatically lock it by selecting the icon on the Create Thread page and selecting Lock. Once posted, the thread will already be locked.
There are two methods you can use to lock an existing thread.
When viewing a thread, click on the Actions dropdown menu on the top of the thread. Select Moderate, and then select Lock from the following sub-menu. The thread you're viewing will instantly be locked.
Alternatively, when viewing the list of threads for the board you're in, you can select the checkbox next to every thread you want locked. Click on Actions, then select Moderate and then Lock in the following sub-menu. This is an easy way to lock multiple threads at once.
These same steps can be followed to unlock a thread. The menu option to lock the thread will simply be changed to Unlock.