It's a general helper library to handle environment variables. You have helpers to handle a "flat" configuration to parse easily integer, float and so on, but you have helpers as well to store a config in your environment variables like you would do with a yaml file. For instance thoses two variables, "CONFIG_DB_PORT=3306" and "CONFIG_DB_HOST=localhost" will produce a tree CONFIG -> DB -> [PORT: 3306, HOST: "localhost"], you have some convenient functions to browse the tree and get datas but you can also dump this tree into a struct. This release add the ability, when the struct is populated, to custom the data validation or to populate some fields with your own logic, have a look here : https://github.com/antham/envh#example-with-a-tree-dumped-in-a-config-struct and you have examples in godoc as well.
It's just a sum up, I forgot a bit of information, I did it in a hurry, but you also need to have some context to understand by looking at the repository. There are several examples provided in documentation to understand what's going on.