Spacetime-6 is a 6-dimensional continuum composed of 3 dimensions of space and 3 dimensions of time.
The three dimensions of space are those familiar to us: the x-y-z axes of a rectangular coordinate system. The three dimensions of time can be thought of as analogous.
The spatial direction of forwards/backwards.
The spatial direction of left/right.
The spatial direction of up/down.
The time dimension of forwards/backwards allows movement into the future or past. It changes your calendar date. (Illustrated in pretty much any scifi time travel episode.)
The time dimension of left/right is like stepping sideways across to neighboring parallel universes. The farther you move, the more reality diverges. The calendar date remains the same, but reality changes. Called parallel universes, quantum realities, or divergent worldlines. (Illustrated in TNG: "Parallels" and any Stargate SG1 episode dealing with the Quantum Mirror.)
The time dimension of up/down means venturing away from the plane of reality altogether into the realm of thought, dreams, and nightmares. The farther you travel up or down from Now/This, the farther you recede from actual reality into the Imaginary. Again, the calendar date stays the same, and your quantum universe doesn't change, but the Real becomes Imaginary. (e.g. TNG: "Where No One Has Gone Before", TNG: "Journey's End".)
Combining motion in the three time directions allows a traveller to move to any time, in any quantum parallel universe, and in any way imaginable in dreams.