Word frequency.
Without knowing the most common words, it is impossible to speak, understand and write in a given language. For example, in German first 30 word forms cover 32% of words of a text. 100 first word forms - it's already 48% of a text.
If you know first one thousand words most used in English, it's already 78% of a text. But then, you can recognize some more words, because they are either proper names (like John or Vietnam) or international words used in many languages. So with 1000 most used words, one can already understand very much of a language. With knowing first 5000 words, one understands about 99% of the text - and it's possible to guess one word in 100.
Of course, the amount of words needed to learn depends on the language : English has plenty of words in common with Spanish, but very little in common with Chinese. But generally, it's better to concentrate on most used words. That's why on this page, there are lists of most common words for different languages, together with translation and many examples.
Most common words in different languages.