L$ is Linden dollars, the in-game currency. You get paid a weekly stipend as a member which you can spend in the Second Life World (Linden). You can earn more by selling stuff you have made (hence the poser bit, or textures etc) or joining the huge number of events. You can even convert the Linden dollars you have earnt into real live American dollars (not a 1 to 1 exchange rate I might add though).