The DSL modem must always be connected to the phone line. It is the "base station" for your internet access. Think of it like the "base station" for a set of in-home cordless phones ... you have a base station which is always connected to the phone line and cordless handsets that you can then roam around your house talking on the phone.
In this case, "wireless" means your computer does not need to be connected by a wire directly to the modem -- such as a laptop or tablet -- you can roam around your home and be connected to the internet. Or, you can directly connect a computer / desktop to the modem as well.