When the governments globally were in Great Financial Crises GFC during 2008 and spent almost $1 trillion to rescue their banks from collapse. At that time the term bail out was widely used and became common.
Bail in is considered as the antidote to bailout because it was coined after the crisis by bankers who wanted to assure the public that the biggest lenders could survive without any more taxpayer handouts.

What’s a bail-in?

A bail-in forces investors in a bank’s bonds when a lender defeats or does bankruptcy.
The term bail-in has come to cover every case of creditor loss-sharing when a bank gets in trouble, although basically it was a part of quick resolution methodology
The bail-in approach was invented in 2010, when executives at Credit Suisse Group AG proposed it as a mechanism to replace bailouts.
The U.S. and the European Union later included the concept in new laws.
When the equity of shareholders is completely wiped out, the banks usually move towards the bankruptcy. It occurs when loans or investments they’ve made go sour.

Banks give shares to the investors of the bank in a debt-for-equity swap as a return a cut value from their bond, it is known as writedwon.
Write down helps bank operations active for a while at least and it is equivalent of a fresh capital.
When you bail in the creditors, they become new shareholders of the bank while it goes through a resolution process similar to bankruptcy.
While bailing in the creditors, they turn to new shareholders of the bank and this process takes place through a resolution which is same as bankruptcy.
It’s less disruptive because the bank can continue functioning with the fresh capital from the creditors.

What’s the case for bail-in?

It was strongly argued by bankers and regulators that banks are unable to handle a regular bankruptcy process because their assets lose value extremely fast.
A resolution headed by a regulator which keeps bank operational can help in preventing a loss of value
The continues funding us usually required by banks which helps them in maintaining their assets and a bail in gives a fresh equity to fill the gap.
Creditors who are bailed in benefit if assets can be sold in an orderly fashion.
When bondholders are hooked, losses created by bailouts will also be minimized.
Moral hazard refer to an idea that banks will take greater risks if they assume the government will step in should things go wrong.