Phase is the relative position between 2 waveforms.
Delay is the adjustment to align the waveforms better.
I know some of you get it, but for those that don't yet here is an analogy.
Two cars are at the starting line of a race. We want to end the race in a photo finish. One is a dumptruck. The other a ferrari. We have to let the dumptruck get a head start, because it is very slow to leave the line. The Ferrari will catch the dumptruck quickly, so we have to delay it leaving so it wont cross the finish before the dumptruck.
Lets say we let the dumptruck leave too early. It finishes a lap ahead of the ferrari, but still crosses the line at the same time. The dumptruck is now one phase rotation ahead of the ferrari. If we get it right, both vehicles will cross the finish at the same time on the same lap. This is what we want. In phase and in time.
We want to properly sum waveforms so that their impulse is at maximum amplitude and shortest duration. ie. in phase and on same cycle.
Using the high level inputs is like attaching a towbar between the ferrari and dumptruck. The ferrari tows the dumptruck at a set distance. Using the phase knob extends the length of the towbar. We cannot get them to photo finish. The best we can do is extend the towbar to one length of the track so the vehicles cross at the same time, but are one cycle apart.