Tao -- also spelled Dao -- has several interrelated meanings:
On the one hand, it can refer to the origin or fundamental principle of all of creation, all of the worlds and universes within the cosmos. It can also refer to the totality, the sum total, of all-that-exists: a kind of universal truth or ultimate perfection that encompasses and is the essence of all that appears.
In a slightly different inflection, Tao can refer to the path or the way that leads one to an experiential encounter with this totality, this supreme perfection; and/or to the specific teachings or techniques that support one in walking this path.
Ultimately, the path/way is found to be not separate from the "destination" -- at which point these two definitions merge into one.