What is 'parameter convergence'?
A naive definition of Parameter convergence is when the weights or the values of the parameters reach a point asymptotically. What I mean is that, when your model training is not altering the parameter values(maybe less than epsilon-small values) it might be a good fit. For decision trees, I found this paper which explains rate of convergence and more. It might be a good read if you want to get more details.