Basically a pile of seats at £4.99 - includes taxes etc - only optional extras will increase your price. The usual guide to follow..
Personally, I found some of these yesterday (Leeds to Alicante) so there might be more than the advertised destiantions available. If it comes up with all the extra taxes etc - move on - it's not in the deal.
You will also find some flights at other "inclusive of taxes" prices, have a mooch around.