Single click on any file to highlight then press <Apple> i or go to File -> Get Info to display the Info window. Look for the General heading. If the small triangle to the left of General is pointing right, click once to open the full General info panel. Near the foot you should see the locked status. This is a toggle. Click once to unlock and click again to lock.
(Triangles against folder names are common in Filer displays when List View is used. They perform the same function as above in that they open the folder display.)
Fred