.gitignore will prevent untracked files from being added (without an add -f) to the set of files tracked by Git. Following the simplest example of a gitignore file.
 

The following example shows that how wen ignore a file or a folder using git ignore 

Add a tracked file to gitignore:
When any file tracked by git and you add it to git ignore However, Git will continue to track any files that are already being tracked.

To stop tracking a file, we must remove it from the index

git rm --cached config.php

To remove a folder and all files in the folder recursively:

git rm -r --cached assets