What do you mean when you send a message to a boy, he just doesn't reply?

It depends on whether this boy is your boyfriend or an ordinary friend. If you are a boyfriend, he always doesn't reply to your messages, which shows that your feelings should be relatively weak. If you are an ordinary friend, it means that this boy may be really busy and have no time to reply, or the message you sent is not particularly important, but if you are a boyfriend, you need to have a good talk with him to confirm the future direction of emotional development.