When you sign up you choose up to 5 interests. Matching stays random, so nobody sits in a separate room waiting for one subject, but people who share one of your interests are preferred when you are paired.
Once you are matched, the interests you have in common appear at the top of the chat, along with a few suggested openers about them. Tap one to send it, and neither of you has to think of a first line.
If nobody online shares your interests at that moment, you can still be matched with anyone else who is waiting, so picking a niche topic does not narrow who you can meet.