git checkout -- <file_name>
Adds files in the to the staging area for Git. Before a file is available to commit to a repository, the file needs to be added to the Git index (staging area). There are a few different ways to use git add, by adding entire directories, specific files, or all unstaged files.
Usage:
$ git add <file or directory name>
In Practice:
# To add all files not staged:
$ git add .
# To stage a specific file:
$ git add index.html
# To stage an entire directory:
$ git add css