Those are documented here, I'll copy pasta though:
lower (optional, defaults to 0) - An
attribute specifying the lower joint
limit (radians for revolute joints,
meters for prismatic joints). Omit if
joint is continuous.
upper (optional, defaults to 0) - An
attribute specifying the upper joint
limit (radians for revolute joints,
meters for prismatic joints). Omit if
joint is continuous.
effort (required) - An attribute for
enforcing the maximum joint effort
(|applied effort| < |effort|). See
safety limits.
velocity (required) - An attribute for
enforcing the maximum joint velocity.
See safety limits.
Effort is between 0 and 100 (units are %) and velocity is radians per second for revolute and continuous joints and meters per second for linear or prismatic joints.