A little phone confusion

Ok well I'm really confused right about now. My friend 4 days ago sent me a text saying 'Sorry but I can't text after this. No money left'. What does that mean? I'm really confused and I miss texting him because he's a lot of fun to talk to. So if you can shed some light as to what this means, please do. Does it mean that he can't text anymore period or does he have to put more money onto his phone for texting? Its confusing because I have unlimited texting and have never heard of someone not being able to text because there is no more money.