Taxable interest would be any interest that you received/earned from a bank account, CD, mortgage that you hold, personal loan that you hold, etc.
Banks are required to mail you a form 1099INT if they pay you 10$ per year or more in interest. However, if you do not get a 1099INT and you earned interest, it is still your responsibility to report it. You should review your year end or December bank statements to look for year to date interest. If you hold any loans yourself, then you should have a record of payments made to you, and how much of the payments represent interest.