lead in is the table of contents of the disc
it tells the reader what format the disc is in, and where all the files are located
the lead out is just blank, and was really created for audio cd-players, so that if you were fast-forwarding through a disc at a fast speed, and got to the end, but the disc failed to stop immediately, it wouldn't damage the player
it's basically just a 3:00 track of nothing but "stop playing!" messages
if you're writing a data disc, you can do without the lead out, so you can actually "overburn" a disc and write extra data (around 30 MB) where the lead out should be.