An income verification letter is a formal letter that your employer fills out and then sends back to a loan company or bank etc for proof of your income.
So if you work at a job then the bank or other loan company will usually contact your employer and want them to fill out an income verification letter which will then either be mailed back to them or faxed back to them.
Once the bank or other loan company gets the income verification letter and proves your employment status and income then they decide whether or not to approve your loan.
You should not have to do anything to get the income verification letter but sometimes you do and if you do need to request an income verification letter you just ask your boss or whoever is in control at the job such as a manager and they should be able to get you an income verification letter.
It's just basically a letter that is filled out to prove your income and job status.