It's a slangy word meaning a little, as in "he was a tad angry". Though now very much used in Britain, especially by journalists, it was originally an Americanism, and my impression is that it wasn't much used in the UK until about 10 years ago.
I don't think it's an Americanism, and I certainly knew the word growing up with family from Ulster, so I guess it may be a Northernism. I do remember a sixth-form English teacher of mine back in 1992 (and down South in Hampshire) not knowing the word and all of us in the class thinking this was very strange, so it wasn't just me and my family.
It has been in use in America since the 1960's. It derives indirectly from the word 'tadpole' which means immature frog. Then people began referring jokingly to a small child, especially a boy, as 'tadpole' or 'tad'. Finally it became used as a synonym for 'little'. It was used by the U.S. humorist/cartoonist Walt Kelly.
monkey puzzle, tadfish from neighbours? nah! tad as in tadpole as toadfish is his older brother. toad? tadpole? see?
serial drama? nah! conversely that would make eastenders a sitcom! try soap.