Meaning:
“自带” means “to bring one’s own …,” and “buff,” in this context, refers to “enhanced ability” in games such as “League of Legends,” where a system of “buff points” is introduced to reward good performance. Therefore, “to bring one’s own buff” is a joking way to say that someone is very talented or skillful in certain aspects; however, it is not restricted just to the context of playing games.
Example:
A: 听说你表哥自由泳比赛拿了全市第一名？
Tīngshuō nǐ biǎogē zìyóuyǒng bǐsài ná le quánshì dìyīmíng？
Is it true that your cousin won first place in freestyle swimming in a citywide competition?
B: 是啊，这小子自带buff，他父母都是旱鸭子。
Shì a，zhè xiǎozi zìdài buff，tā fùmǔ dōushì hànyāzi。
True. The kid is talented. None of his parents can swim.