A subtweet is a tweet that is about a person but doesn't mention their username with the @ symbol. For example, if someone tweeted "Hey computerhope I need help" we would not be notified because it's missing the @ symbol in front of our username. However, we would be notified if the tweet was "Hey @computerhope I need help."
If your username is unique enough, you could also use the Twitter search to find any tweet containing your username.
What is the character limit for a tweet?
Messages on Twitter can be a maximum of 280 characters.