The Path is a term witchers use to describe traveling the various parts of the world hunting down monsters. Witchers travel the Continent looking for contracts to fulfill, there is no stated limit to what constitutes a contract; a witcher could be tasked with killing a werewolf, eliminating a nest of drowners, or investigating the mysterious deaths of people at the hand of a potential monster. Towards the onset of winter, witchers reconvene at their respective keeps to wait out the cold season. Vesemir from the School of the Wolf states that wintering at the witchers' keeps is usually done by witchers who have been unable to integrate into society.
As described in The Witcher 3, they'll wait out the winter and once spring comes along they're back out plying their trade. Witchers are trained to work alone but on occasion have been known to partner up on a contract and split the reward.