In English, we use "hyphen" (punctuation mark) to join two or more words. These should not be confused with "dashes", who are a longer punctuation mark and have other uses like separate explanatory phrases.
Hyphens are used to join words or terms to avoid confusions and they are written with no space between them and the words. In some cases, they are used to preserve the text's clarity: when adding a prefix (great-grandmother) or when the final and initial letters coincide (co-operate).
In this specific case, "two-thirds" is a fraction, so the hyphens are used to let know the relation between these two words instead of reading them as separated numbers.
