R, xgboost: eval_metric for count:poisson
This is from XGBOOST documents:
“poisson-nloglik”: negative log-likelihood for Poisson regression.

You might want to go and read more about poisson n loglikelihood.

Here is a link for you to get started:
https://onlinecourses.science.psu.edu/stat504/node/31

Hope this can be helpful