The .gitignore file tells Git not to track certain files in the project. For WordPress, here is a recommended .gitignore file you can modify for your project. For more on the mu-plugins directory included in the file, see the linked guide.
# -----------------------------------------------------------------
# .gitignore for WordPress
# -----------------------------------------------------------------
# ignore everything in the root except the "wp-content" directory.
/*
!wp-content/
# ignore all files starting with .
.*
# track this file .gitignore (i.e. do NOT ignore it)
!.gitignore
# track .editorconfig file (i.e. do NOT ignore it)
!.editorconfig
# track readme.md in the root (i.e. do NOT ignore it)
!readme.md
# ignore all files that start with ~
~*
# ignore OS generated files
ehthumbs.db
Thumbs.db
# ignore Editor files
*.sublime-project
*.sublime-workspace
*.komodoproject
# ignore log files and databases
*.log
*.sql
*.sqlite
# ignore compiled files
*.com
*.class
*.dll
*.exe
*.o
*.so
# ignore packaged files
*.7z
*.dmg
*.gz
*.iso
*.jar
*.rar
*.tar
*.zip
# ignore everything in the "wp-content" directory, except:
# "mu-plugins" directory
# "plugins" directory
# "themes" directory
wp-content/*
!wp-content/mu-plugins/
!wp-content/plugins/
!wp-content/themes/
# ignore these plugins from the core
wp-content/plugins/hello.php
wp-content/plugins/akismet/
# ignore specific themes
wp-content/themes/twenty*/
# ignore node/grunt dependency directories
node_modules/