In about 1000 C.E. the Ashkenazic halachic authority Rabbeinu Gershom of Mainz issued a decree called "Cherem de'Rabbeinu Gershom" banning bigamy. To prevent this decree from causing flippant divorces previously unnecessary, he also decreed that "a woman may not be divorced against her will."
In certain extreme cases, however, such as the case of a man whose wife is missing, or refuses to accept a get for an extended period, Beth din will only permit him to remarry after one hundred rabbis agree with them to issue an exemption.
After receiving a heter meah rabbanim, Beth din will require the husband to write a get for his wife and deposit it with them. His wife will remain married until she receives the get in her possession.
To insure that a particular situation indeed justifies an exemption, the rabbis instituted a requirement, that at least one hundred Torah scholars domiciled in at least three different countries or, according to some authorities, it is enough three different jurisdictions, certify that dispensation for a second marriage is factually justified.
In order to get a heter meah rabbonim, it used to be that a man who got the go ahead from a Rabbinic court wandered from town to town and from one country to another with a letter from Beth din and had to plead his case with every town Rabbi to get his approval. Later on, written permission by mail was accepted and sometimes an intermediary was used to plead his case. In the last century with the ease of communication, it has become a more formal process in which Beth din takes the lead and secures the one hundred signatures required.
Situations where Beth din might see a justification which warrants this process includes: