They measure two different properties, so you need to follow both of them (and they don't always agree). pH is a measure of how acid or alkaline a solution is. The scale runs from 0.0 (as acid as water can get) to 14.0 (totally alkaline). Neutral water is pH 7.0. It is a logarithmic scale, so a change of 1 pH means the concentration of Hydrogen in the mix goes up or down by a factor of 10X. Most living things don't generally tolerate anything too far from pH 7.
PPM just tells us how much total stuff is dissolved in a sample of water. The number doesn't tell you WHAT is in that water, just how much. Zero PPM means absolutely pure water, 1000 PPM would be a fairly stiff mixture of most fertilizer mixtures, for example. If your house has hard water, the PPM reading will include that as well as whatever you intentionally add.