Hello,
the 'Bahn Bonus Card" is a customer loyalty program of "Deutsche Bahn", you can use it as a common or as kind of a premium customer (BahnCard User). After you have registered (****://bit.ly/2axlQsM) you can collect bonus points with any train ticket you buy. If you have got enough points you can choose a bonus gift from a bonus catalogue (****://bit.ly/2axkA8W).
It's a respectable program but it's only worth to register if you travel quite often by train.
Kind regards, Jan