The path/line of least resistance describes the way all forces or objects in nature always travel along the path that requires the least amount of effort or force, or, offers the least amount of opposition. It is also used metaphorically.
In the markets, if there is more demand than supply the path of least resistance would be up. If there is more supply than demand the path of least resistance would be down. You wouldn't 'buy' the line of least resistance, you would first try and determine which direction (path) offers the least resistance and then trade in that direction.
It's a typical bull**** line that bods throw around when they don't have a clue, but want to sound clued.
If the price is heading down, the least resistance is obviously downward, (if you want to draw a line, do so). In that situation, you wouldn't really want to buy it, would you?
Switch gears and figure the reverse for a price heading upward.
Moving away from trend specific ‘lines’ of resistance (or lack of), use any flavour of Support and Resistance you care to shake a stick at. In this case, least resistance (or Support) means the price carries on through. When it gets to an SR level and stops and looks kind of uncomfortable. That’s a clue. It’s now potentially offering Resistance (or Support).