Your onset date is that date that you became disabled. For a lot of you, that is the day that you stopped working. It could also be the date of something significant, such as an accident or a surgery; something that happened and afterwards, there was no possible way you could work.
Your onset date becomes important because, if you are found disabled by the Social Security Administration, that is the date that they begin paying you benefits (for Title 16 benefits) or they begin the 5 month waiting period (for Title 2 benefits).
If you are having a hard time coming up with a date, it might help to talk to your doctor. There could be something revealing in your medical records that your doctor can point to.