Tab/practicing Question

Hey Andrew, I just had a small question about the tabs, so I figured I'd ask it here. What does it mean when you put an "L" on a tab...for example, from Gabriel's Alice in Chains style lesson:



E|-----------------------|-------------------------|
B|-----------------------|-------------------------|
G|-----------------------|-------------------------|
D|-----------------------|-------------------------|
A|-------4-------3----2-|--L--------5------------|
E|--2---------2---------|-----2--2--3----0--2--3-|


oh...and another question...I know a few licks and songs by heart, that I know very well...but I can never seem to be able to play perfectly every time..even though I've been practicing them for a while...and then I see some of these videos and I wonder if you guys record over and over again to make it absolutely perfect or if there's some trick to it. If I mess up even once I get all confused, and have to start over again...

Just a few simple questions, thanks for your time.