what does regularization mean in xgboost (tree)

In xgboost (xgbtree), gamma is the tunning parameter to control the regularization. I understand what regularization means in xgblinear and logistic regression, but in the context of tree-based methods, I'm not sure how regularization works.

Can someone explain how regularization works in xgbtree?

This minimum gain can usually be set for anything between $$(0,\infty)$$.