Tell me about it
Definition 1 of 2
Pro Tip 1/3Shared Experience
Use it when you have experienced similar situations and understand the feeling. 
A: This traffic is a nightmare every day. B: Tell me about it, it takes me an hour to get home.
Pro Tip 2/3Sympathy Expression
Conveys empathy, indicating you sympathize with the speaker's situation or sentiment. 
A: I can't believe the price of groceries lately. B: Tell me about it, my wallet is crying.
Pro Tip 3/3Colloquial Agreement
Informal way to strongly agree with someone without needing to elaborate. 
A: These exams are killing me. B: Tell me about it!