This is an easy one - buy one get one free! Many bingo sites have BOGOF deals on their bingo tickets - buy 6 bingo cards and get another 6 free. Just like buy one get one free at the shops!
The BOGOF offers will come in all different shapes and sizes. Some BOGOF offers will be on every online bingo ticket and some with a minimum ticket purchase of 3, 6 or even 12 cards. Many bingo sites will have BOGOF deals at varying times of the day or week or even month. Then you have buy one get one free deals in one bingo room, some rooms or even all rooms! Or, some sites think BOGOF deals are excellent and offer them 24/7 in all their rooms!
The best thing you can do is check a UK bingo site's individual promotions to see if/when they offer BOGOF deals, but at least you now know what BOGOF stands for!