I've always found "Buy low and sell high" to be one of the most infuriating axioms in trading (up there with "control your emotions"), and not because it's not true.  It's because it's vague. 

Well all know what it means to buy low, but "low" is subjective,  and we all know that we want to sell for a profit,  but for how much profit?  A couple ticks up in a profit, but then again so is a couple hundred ticks.  I can't tell you how many times in my early trading days I froze up on buying a dip because "What if it keeps going?  What if I'm wrong?"  or I convinced myself I could get a better entry and I knew better than the markets did.  On the flipside, I can't tell you how many times I saw the market get close to my take profit point only to see it reverse and either take me out for less than I could have had if I were being flexible and alert or, in some cases, for a loss. 

The worst part of this axiom, though, is that it's so simple it's easy to convince ourselves we get it which can be even more dangerous.

Personally, I describe it to new traders as "Buy value, sell weakness" 

It still makes sense to most people but it requires questions like "how do I spot value and weakness?" before it can be implemented, and that is when the real learning can start.