The phrase suck it is basically a nicer way of saying to fuck off or bite me as they don't care what you think about them or something the person has done.
For example you don't like my clothes that I'm wearing then suck it. Which means that I don't care if you don't like what clothes I'm wearing.
So suck it.
Or you don't like doing your homework then suck it because you have to do the homework.
The phrase suck it can also mean to Literally: "Suck my genitals." when some people say it.