What is the appointment between the two friends about in "After Twenty Years" by O. Henry?

The appointment between Bob and Jimmy to meet again in twenty years was just for the sake of reunion. It was similar to the thousands of class reunions, family reunions, and all the other reunions which are so common in America. People enjoy seeing each other again after the passage of time and hearing what has happened to those with whom they used to be close. Bob is going to leave New York the next day. He and Jimmy have been friends since early childhood. Both hate to think they are going to be parting forever, so they make a sentimental agreement to meet again at the same place, 'Big Joe' Brady's restaurant, in exactly twenty years. It may have been a mistake to make such an appointment; Bob and Jimmy didn't realize how much people can change in that length of time. If they had actually been able to sit down at a table and talk, they probably would have found out they had nothing in common. It would have been a disappointment with many awkward lapses in conversation. The men might have been glad to separate again, and they probably wouldn't have made another appointment to meet again in ten or twenty years. One thing Bob discovered was that 'Big Joe' Brady's, a seemingly permanent institution, had been torn down five years ago. 'Big Joe' himself was probably dead.

Bob tells the cop, whom he doesn't even recognize as his old friend Jimmy, how the appointment came to be made.

“Twenty years ago tonight,” said the man, “I dined here at ‘Big Joe’ Brady's with Jimmy Wells, my best chum, and the finest chap in the world. He and I were raised here in New York, just like two brothers, together. I was eighteen and Jimmy was twenty. The next morning I was to start for the West to make my fortune. You couldn't have dragged Jimmy out of New York; he thought it was the only place on earth. Well, we agreed that night that we would meet here again exactly twenty years from that date and time, no matter what our conditions might be or from what distance we might have to come. We figured that in twenty years each of us ought to have our destiny worked out and our fortunes made, whatever they were going to be.”

The reader can imagine what a place like 'Big Joe' Brady's was like twenty years ago — a spacious room full of noisy men drinking beer and all talking and laughing at once. The setting has changed with time. 'Big Joe' Brady's space is now a closed and darkened hardware store in a silent neighborhood. Bob has to stand inside the doorway because of the wind and rain. The contrast between the old and the present settings symbolizes the ravages of time. Everything changes. Bob was just eighteen and Jimmy only twenty when they made that appointment. They assumed that the restaurant and the neighborhood would still be the same, and they assumed that their friendship would still be the same. They were too young to know nothing ever stays the same.