I do not know the exact Bengali word. But let me try to explain you the meaning. Here: Looking to get free mobile top-up with no catch? Sign up to get free airtime. means they are providing a free mobile top-up with no other condition attached. Sometimes this kind of free offer is related with some hidden cost, I mean when they say it is Free but actually when the user go to avail the same realize that it is not free but connected with some other purchasing activity. I mean you will have that free thing in case you by something else. But here no catch means: it is simply free, not liked with anything else.