关于 means "about" in
Chinese. The word which is put after "关于" is related to the second sentence.
- 关于这个问题我们下次再说。 In this sentence, the subject we talked about next time has something to do with "this question".
- 关于这次实践活动,你最好写个简单的总结吧。 In this sentence, the summary you write is relevant to "the practical activities".
根据 means "according to" ''based on''. The matter which is put after "根据" is the basis of the second sectence.
Furthermore, 根据 is a noun sometimes, and means basis.