Hullos are short instant messages you can send to your friends and family, for free, no matter which version of the app you have. It's like leaving someone a voicemail, but without the initial phone call.
The recipient doesn't need to be a HulloMail user, although it would be great if they were! You only need someone's email address in order to send a Hullo message.
Your mobile operator may charge you for sending Hullos if you have exceeded your monthly data allowance, or if you are roaming.