While people are layin, many times they listen to music, talk to friends, lay with pets, spoon, or for the full effect they do nothing at all.
EX: In the summer I'm always layin in the AC with my puppy.
EX: When you hung out on your bed all day and your friends asked you what you did all day you might respond "I was layin."
Lay = the actual action
Lay Sesh = The period (or session) in which you were layin