From the topic of your post I'll assume the surgery was done on an anal fistula. There are a few different proceedures you can have for this, but the common theme for most of them is that you are left with an
open wound that will need to heal to close up the fistula. In those cases, drainage of pus or even some blood is normal. Typically you would place some gauze in your underwear for a couple of weeks after the operation to catch the drainage - and in most cases you will have a follow up booked with your surgeon so he can check up on the healing.
Things to watch out for are generally increased pain, redness, swelling, or other signs of infection in the area.
However, given that I don't actually know what proceedure was performed on you, I think you should call your surgeon's office and check with him that the drainage is expected and ok.