Threescore means sixty. It is made of "three" and "score," and a score is twenty, so threescore is three twenties. This is an old way of counting that English used to use a lot, the way we might say "three dozen" for thirty-six today. Nobody counts like this in ordinary modern English — you will not hear a shop assistant say "threescore eggs." You meet the word in three places: the Bible ("threescore years and ten" for a human lifetime of seventy), old poems and speeches, and jokes or writing that is copying that old style on purpose. It is often followed by "and ten" or "and five" to make the numbers in between: threescore and ten = 70. When a modern writer uses it, they usually want the sentence to sound solemn, ancient, or a little grand. So the word tells you as much about the SPEAKER as about the number.

六十（古語說法，「三個二十」）