You can use it to express the fact that you don't care about another's sadness or unhappiness. You can also use it to refer to the act of crying excessively in front of somebody. If your partner broke up with you then complained about being bored, you could reply, "cry me a river." If a child was having a tantrum because he or she didn't want to go to school, mother might reply, "you can cry me a river, but you're still going to school."

Here's an article about this expression: https://www.theidioms.com/cry-me-a-river/

basically, it's used sarcastically when someone is upset or sad, and you couldn't care less about what they are feeling, like if someone is crying about something that is stupid or doesn't have a justification to be upset or making a bigger deal than it is. you would say "cry me a river" say if your partner left you for someone else and came back and begged to get back together. you could say cry me a river. or say your friend is crying because their yard worker couldn't come that week you could be like "oh cry me a river, I don't even have a yard worker"

Like Will said, you don't care. The person could cry enough tears to make a river and you still wouldn't try to calm them down.

it means J.T. :D