A lytic lesion is an area of damage on a bone. The damage might be just a weakening of the bone or perhaps a small hole in the bone. It is called a "lytic" lesion because it is caused by a process called "osteolysis". This is the process by which cancer cells metastasizes, meaning changes and spreads.
A lytic lesion in the L2 verterbral body refers to a weakened area of bone in the lumbar region of the spine - the L refers to lumber, the 2 refers to the second disc of that region. The lumbar is the lower back.