What is succession certificate ?

Respected Sir/Madam, I am a 40 year old woman from Thane. I had purchased my flat here in 2003. It was owned by me and my husband in joint name. But after sudden death of my husband it later got transferred to my name only. I have a daughter who is 20 years old and a son who is 13 years old. Now, I am selling the flat off to a decent party and they are purchasing it through a loan from the bank. The bank to which they have applied for a loan is asking for a succession certificate from me because my son is a minor. All other formalities have been duly completed apart from this. It takes a lot of time to acquire the succession letter and my money is going to get stuck in this matter. Due to financial crunches, I will not be able to wait too long. I had also submitted an affidavit with notarization to the bank. My question to you is- what is succession letter? And why is it needed when the flat was already on joint name ? What is the use a joint ownership then ? I will be obliged if you will explain my case in brief. Thank you.