What does 前方高能 mean?
https://baike.baidu.com/item/%E5%89%8D%E6%96%B9%E9%AB%98%E8%83%BD

前方高能一般认为最早出自《机动战士高达》系列。指宇宙舰队在太空行军过程中为了安全，会时刻对前方的宇宙空间进行能量侦测。若有高能量反应，则及时规避。
而在各ACGN弹幕网的视频弹幕中经常会出现“前方高能”之类的弹幕，则是预示接下来会出现激烈的内容或画面。

It is apparently a Japanese phrase commonly seen in Japanese anime.

&quot;Front high-energy&quot; is generally considered the earliest from the &quot;Mobile Suit Gundam&quot; series. Refers to the space battle ships (or mobile suits) travel in space, for safety reason, will always sweep for high energy reactions. If detected a high energy reaction, The pilot can decide to take evasive action or prepare to engage enemy.
In the videos from ACGN network.  The message  &quot;前方高能&quot; (High energy reaction detected in front) would often appear on screen. Whenever it does, the two enemies are in a collision course, intense battle will begin shortly.

Nowadays, they also use this phrase to announce upcoming exciting sequence of all kinds, not just for battle