Cannot checkout, file is unmerged

If you want to discard modifications you made to the file, you can do:

git reset first_Name.txt
git checkout first_Name.txt

status tell you what to do.

Unmerged paths:
  (use "git reset HEAD <file>..." to unstage)
  (use "git add <file>..." to mark resolution)

you probably applied a stash or something else that cause a conflict.

either add, reset, or rm.

Following is worked for me

git reset HEAD

I was getting following error

git stash
src/config.php: needs merge
src/config.php: needs merge
src/config.php: unmerge(230a02b5bf1c6eab8adce2cec8d573822d21241d)
src/config.php: unmerged (f5cc88c0fda69bf72107bcc5c2860c3e5eb978fa)

Then i ran

git reset HEAD

it worked

To remove tracked files (first_file.txt) from git:

git rm first_file.txt

And to remove untracked files, use:

rm -r explore_california