When using the verbs wait, stand, or be, the article "the" is generally unnecessary, so we say "wait in line", "stand in line", or "be in line".
When talking about a row of people or things, you can say "it's a line", "it's a short/long line", or "the line is short/long".
The line is too long.
However, when using the verbs be, stand, wait, or get, the article "the" is generally unnecessary, so we say "be in line", "stand in line", "wait in line", or "get in line".
She doesn't like to wait in line.
Yesterday I had to stand in line for three hours.
Are you in line? Yes, I'm in line.
The expression "cut in line" (without "the") is commonly used when someone goes in front of other people who are waiting in line.
Hey, don't cut in line!
We get annoyed when people cut in line in front of us.