"I should have bought the tickets already" 意思是 "我早该买票了"。

这句话的来源是英语中的动词 "should"，它表示"应该，本来"。例如：

You should brush your teeth before bed. (你应该在睡觉前刷牙。)
She should have finished her homework by now. (她本来应该已经完成作业了。)

"Have bought" 是动词 "buy" 的过去完成式，表示在过去的某个时间已经买了东西。例如：

I have bought a new car. (我已经买了一辆新车。)
They have bought tickets to the concert. (他们已经买了音乐会门票。)

所以，整句 "I should have bought the tickets already" 的意思是 "我早该买票了"。