In Twitter, the hashtag or pound sign (#) is used to mark tweets by topic. The concept is simple: label something so you can easily find it later when conducting a search.
You can define your own hashtags or use one that has been defined by others. Sometimes event organizers and groups will define the hashtag as a way to aggregate all the tweets on an event or about a specific group.
Twitter search can be used to find all the content with the same tag.
There are no rules for hashtags. Literally anyone can use one at any time for any meaning. Like may things with social media, this freedom can be both a blessing (all I have to do is declare a tag and poof, it’s mine) or a curse (what does that tag mean?!?).
In the meantime, I recommend you use them whenever participating in a group or event to make it more likely that your comments are seen by others. Use them to group other tweets if that helps you, and doesn’t confuse your followers.