" To know (something) by heart" means to have something memorized, to be able to say it without reading it.

“to know (something) by heart” 是指记住某事， 不用看也能说出来。

When you know something by heart, it means you know something completely; you don't have to study it, you don't have to look at it.

当你记住某件事时， 这意味着你完全了解某件事；你不必研究它， 你不必看它。