Hi, this expression has the same meaning like "thanks or thank you" in English. but we use it in some situations like:
1)when someone is doing something for you and you want to response informally BUT after thanking.
for example: merc, ghorboonetoon beram مرسی قربونتون برم
NOTE: Sometimes we use this expression formally with the same meaning of thank you. If someone says something good or positive about you, in response you could say "mersi, ghorboonetoon beram OR mamnoon, ghorboonetoon beram."
but this time it is formally.
2) when someone wants to show his/her love and kindness to someone.
for example: ghorboonet beram maman.
قربونت برم مامان.
I hope it was useful and clear enough.