What's a "trend"?
You can find the trending topics on the left side of your screen if you're on a computer, or by clicking the magnifying glass or "search" icon on the top tab of your Twitter app. Trends are determined by an algorithm and are tailored to you based on where you are located and who you follow. It identifies topics that are popular at the moment, rather than things that have been popular in the past or daily. This can help you figure out which topics are hot on Twitter and which discussions you may want to jump in on.
If you want to know more about a trend or see what other people are saying about it, just click on it!
Do you want to participate in a trend?
Participating in trends is fun and a great way to gain followers. Just make sure when you compose your tweet that you use the exact word, phrase, or hashtag that is trending. You may not always be able to see your tweet on the trend's timeline due to the vast number of tweets that are posted there but your followers will be able to see it. And when you initially look at a trend, you should be able to see what people who are connected to you are saying about the topic.
Are there rules?
There are a few things you'll want to keep in mind so that you don't abuse your participation in trends. This could cause your account to be filtered or even suspended.
For starters, don't add one or more topics to your tweet that are unrelated, to gain attention in a search. Don't repeatedly tweet the same topic/hashtag without adding value to the conversation or to get the topic trending/higher. Don't tweet to each trend, especially if you're using it as a way to advertise your company/website. Only tweet to relevant trends or ones where you add value to the conversation. Listing trends with a request to follow your account is also frowned upon. Finally, don't tweet about a trend and then post a misleading or unrelated link.
If you really want to play it safe, check out the official Twitter Rules.