Yes, a lemma is the simplest form or base form of a word.

是的，词的是某个单词的最简或最基本形式。

And good to know I just need to learn about 800 words, or what he calls lemmas.

很高知道我只需要学习大约800个单词，或者像他说的那样，词。

For example, with English, I would suggest if you learn the 800 most frequent lemmas-which is a word and its inflections-that will account for about 75 percent of all of the English language.

以英语为例，我会建议如果你学习了800个最常见的词——这的是一个单词以及它的曲折变化——这样就占据了所有英语语言的75%。