Anything you earn outside of employment should be captured. This could be income you have received from selling goods, renting stuff or providing services to your customers.
Side business income does not include:
the income that you receive through your full-time job
any money that you have transferred from one account in your name to another
any income from selling old stuff that you didn't buy with the intention of selling on (what would be considered 'trading')
winnings from gambling, including the lottery, and matched betting.