Its 8 characters (Combination of your name & year of birth) -
First 4 characters are first four letters of your name (In Caps)
Last 4 characters is your year of Birth.
E.g. - My name is Sudhir & my year of birth is 1995, the password would be SUDH1995
Note: Earlier, it used to be the pin number of the aadhar holder. Its not valid for the new aadhar downloads.
If you attach aadhar with another bank's account number, then the second one will show in the Udai site (first in first out). Any subsidy (like gas) will be deposited to the second bank account.