'Jacket weather' is an informal compound noun meaning 'weather cold enough for you to need to wear a jacket when you go outdoors'. In most of the northern hemisphere, you can usually go out without a jacket in summer and into September. But once we get into early October, the weather gets a bit chillier and you need a jacket. Likewise, you might describe the first really warm days of May as 'sandal weather'.

It means that it is cold enough outside to need a jacket or perhaps even a coat to stay warm.