Crossword Clues help.

[Finebyme]

I thought I'd open a thread for anyone who does crosswords, like me who needs a bit of help, I do easy ones, but still get challenged.
Today I have a tricky crossword clue that I can't get it as I am not familiar with darts.
The question is - What is a darts line?


I resist looking it up, as it is more fun discussing and sharing with members I think.