"To drink a glass" means to consume a full glass of a beverage, usually referring to drinking water, juice, milk, or other liquid from a glass container. Full definition
Take, for example, the matter of drinking a glass of wine or beer. (religion-online.org)
A small change can be as simple as drinking a glass of water before coffee, eating almonds instead of a candy bar, or going for a ten minute walk in the afternoon. (insonnetskitchen.com)