The question is getting at the reason for the income and whether it can be treated as a self-employed business on your return as opposed to the income being from a hobby. So the "main job" is getting at whether this income was earned from something you are doing to help you make a living. If it is self-employed contracting/free lance work then self-employment taxes will be assessed, plus business expenses can be deducted.
Therefore, answer the question as you feel best applies -- if the answer is "yes" then the assumption is that this is a self-employed business and self-employment taxes will be assessed, plus business expenses can be deducted. If the answer is "no," you will be asked if this only occurred in 2018 or did you earn the income in the past and/or will it occur again and then be asked whether the intent is to make money. Both lines of questioning help determine whether the income is subject to self-employment taxes and whether expenses can be deducted from the 1099 MISC income.