title: What is the meaning of min_sum_hessian_in_leaf in lightgbm?
body: <p>What is the meaning of min_sum_hessian_in_leaf in lightgbm (see <a href="http://lightgbm.readthedocs.io/en/latest/Parameters.html" rel="nofollow noreferrer">http://lightgbm.readthedocs.io/en/latest/Parameters.html</a>)? I know that the hessian is a matrix of second order derivatives but I don't understand what that means in the context of lightgbm (or gradient boosting in general). And how does lightgbm condense that matrix down into a single value?</p>