well, ph is tthe acidity or alkalinity of a substance, in the case of growing, it usually refers to our water or nutrient solutions..
the ph scale i do believe runs from 1 to 14, the lower the number, the more acidic the solution is.. the higher the number, the more akaline it is..
naturally, cannabis has a preffered ph spectrum, dependent on what style of growing one does, ie, hydro or soil.. for soil, it is a general rule of thumb to keep nutrient solutions and water at around 6.2 / 7.0... or slighly acidic to perfectly neutral, which is what 7 is on the scale..
some nutrients are only available for the plant to use readily in a certain ph range, which is why ph is important.. too high or too low of a ph and it can lock out certain nutrients, which generally means that those nutrients won't be available for the plant to use, regardless of whether or not the solution contains the element or not..
a simple low down on ppm is firstly, it's called parts per million, which refers to the amount of any chemical, ie, nutrient in a water solution.. naturally, the higher the ppm numbers are, this would mean that they are more nutrients in the solution mix, and the lower the ppm numbers, the less nutrients are in the mix..
again, according to the style of growing, different ppm levels are required.. same as for the differing phases of growing, ie, seedling stage, veg stage and flowering stage.. you usually start off with a very low ppm level of around 300 or so for early seedling, and can work up to about 1500 or so depending on the strains and how much nutrients that they can handle..
there are plenty of online ppm guides for each phase of growth for different nutrient companies available that pretty much break the feeding schedule down from week to week or w/e..
hope that helps you out some m8..