His briefs here are the design instructions, not his briefs, like his underpants.

这里的 briefs 指的是设计说明，而不是他的三角裤（briefs 有“内裤”的意思），也就是内裤。

Brilliant's lessons are broken up into bite-size pieces, with instruction designed to guide you through problems.

Brilliant 的课程分为小块，其中的说明旨在指导您解决问题。