"winter" means traditionally the fourth of the four seasons, typically regarded as spanning either the period between the winter solstice to the spring equinox, or the months of december, january, and february in temperate and polar regions of the northern hemisphere and the months of june, july, and august in the southern hemisphere. It is one of many casual words teens pick up from friends, social media, music or sport, and on its own it carries no particular warning. If you hear your teen use it, it is fair to ask what they mean by it in their friend group, since meanings drift quickly. Asking out of curiosity rather than alarm tends to keep the conversation open and useful.