The saying's a rehash of "Buy Low, Sell High" that I remixed from retail theory. I come from a business background, so when I design or evaluate strategies, it gives me some foundation to work on and a measure of success and it gives me a concept I can understand from the get-go, rather than get myself cross-eyed over the latest stochastical adaptive moving ATR of RSI!
It's my "philosophical way" of saying the lower you can purchase something for and the higher you can sell it, the better your margins. It's pretty duh, but I like making simple rules for myself so I can understand better. In retail, it works like, if the market price for a loaf of bread is R10 (selling price), you'll "make your money" immediately when you buy it for R5. You make even more if you buy it for R2. By buying it for R2, you've made R8 at the time of purchase, you get paid when you sell.
In trading, it translates to the type of trading I do - trend following, get in as low as safely possible on an uptrend, and you've already booked a profit, you cash in when you leave at the top. I guess you can apply it to any kind of trading, especially if you've got a set target for a trade. Really simplistic and watered-down way of looking at it, but that's how I like my rules.
I don't know if I did a good job of explaining that. However, it's a rule that works for me. That way, I don't place emphasis on one part of the trade, the entry or the exit, but rather try to get the whole strategy working as a cohesive unit. It takes many line of code to do that