Clock icon on a single WhatsApp chat message on both side

Today, I was talking with one of my friends on WhatsApp and suddenly a clock icon showed on some messages. First, I assumed that there might be an internet issue, so I sent messages to other contacts on WhatsApp and they were delivered. But for half an hour, my friend's chat was still showing a clock sign, and the same issue was faced by my friend. I faced this issue for the first time in my life. Why is that so?