And if you catch someone red-handed, then you catch them in the act of doing something wrong.

如果你当场抓获什么，那就是抓住了他们做错事的把柄。

If you get caught doing something, this means someone saw you in the act of doing something, and it's something you didn't want other people to see.

如果您在做某事时被抓到， 这意味着有人看到您正在做某事，而这是您不希望其他人看到的。