* Ah je sais pas - This means "I don't know". It's a very informal way of saying it.
* dsl - This is short for "désolé(e)" which means "sorry" or "apologies".
* joyeux anniversaire - This means "happy birthday".
Putting it all together:
The phrase is essentially saying: "I don't know (what to say), sorry, happy birthday".
It's a bit of a playful, informal way to wish someone a happy birthday, perhaps when you're caught off guard or haven't planned anything special.
It's important to note that while this is common in informal settings, using this phrase with someone you don't know well or in a more formal context might not be appropriate.