Every newbie gets told to "relax" about a hundred times, it'll take time to come but is important for productive training.
What they roughly mean is:
a) Pace yourself. Don't try to go all out for 30 seconds then be unable to carry on rolling without passing out or throwing up. Learn that gassing sucks and that a purple complexion suits nobody.
b) Don't be so tense. It'll slow you down and make you tire quicker. Not every muscle in your body has to be working at full contraction the whole time!
c) Don't freak out in bad positions or when you're caught in a sub, it's just training. By staying calm and reacting instead of panicking you'll learn more.
d) Expend your energy as efficiently as possible.
e) Don't try to do moves a hundred times faster than needed (or that your skill level allows). Mechanics and leverage are important too
f) Don't try to bully moves. Use what is there, not just what you want. Also, learning when to let go of a move is as important as when to go for one.
g) Head squeezers suck. You're there to learn, not to try and headlock someone to death.