first come, first served 先到先得(非正式)
Truncated form of the sentence "The people who come in first will be served first." meaning that customers are served according to the order in which they arrived.
There are not enough concert tickets for everyone in the long line, and since it's first come, first served, it's possible that the people who are last in line may not get any tickets.
音乐会的门票已经不够长队里每人一张了。先到者先得,所以排在队尾的人可能拿不到任何票了。