One up-one back means that one player is standing at the net while their teammate is on the baseline. This is the most common positioning for the start of a doubles point.
The pattern and positioning changes as the point progresses and may end up with both players back at the baseline, or both players at the net. However, knowing where to stand with one up-one back is crucial to the progress of your doubles.
The base liner is returning serve while the net player is helping call the serve in or out, and then trying to make an aggressive poach during the rest of the point. One up-one back makes poaching (the opponents net player crossing the middle of the court to intercept the ball) more effective because there is a bigger middle opening with this stance, just remember it is also bigger for your opponents to hit as well. Make sure you and your partner are covering that middle!