--hard

This option is considered to be dangerous because it modifies your Working Directory, and basically resets everything to match the commit on which you reset to. So any changes that you've had in the Staging Area and in the Working Directory will be lost (you can still recover from this state and we will talk about that in the Git Reflog section). Let's see an example.

➜  git-basics-beginners git:(master)
 touch git-hard-example && git add . && git commit -m "Add git-hard-example file"
[master e103b63] Add git-hard-example file
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 git-hard-example

➜  git-basics-beginners git:(master) git --no-pager log --oneline
e103b63 (HEAD -> master) Add git-hard-example file
acb1c52 Add git reset --soft explanation
6953063 Add git reset --mixed explanation

➜  git-basics-beginners git:(master) git reset --hard acb1c52
HEAD is now at acb1c52 Add git reset --soft explanation

➜  git-basics-beginners git:(master) git status
On branch master
Your branch is ahead of 'origin/master' by 3 commits.
  (use "git push" to publish your local commits)

nothing to commit, working tree clean

➜  git-basics-beginners git:(master) git --no-pager log --oneline
acb1c52 (HEAD -> master) Add git reset --soft explanation
6953063 Add git reset --mixed explanation

As in the previous examples we added a new file and committed it. After we did git reset --hard our HEAD is now at acb1c52 and git status shows that there is nothing to commit. There is also one interesting notation that you can use $ git reset --soft HEAD^ => here HEAD^ represents commit's first parent you can also use HEAD~1 which represents commit's first parent as well. HEAD^2 or HEAD~2 would represent commit's second parent and so on.