Trading in Rivian Automotive, Inc. (RIVN) was quiet Tuesday, but something interesting happened beneath the surface. The shares appear to have broken through a resistance level that's been keeping a lid on the stock. That could be a bullish signal worth paying attention to.

Let's back up and talk about how this actually works. The stock market runs on supply and demand, just like everything else. When there are more shares for sale than people want to buy, sellers have to drop their prices to attract buyers. The stock trends lower. When there are more buyers than shares available, buyers have to outbid each other to get what they want. The stock trends higher.

Pretty straightforward, right?

Resistance is what happens when you hit a price level where a bunch of supply suddenly shows up. Maybe it's investors who bought at that price and want to break even. Maybe it's traders taking profits. Whatever the reason, there's suddenly enough supply to match or overwhelm the demand. Rallies stall out or reverse at these levels because buyers don't need to keep pushing the price up anymore.

But sometimes the supply dries up. The sellers finish their orders or cancel them. If buyers still want shares, they have to start bidding the price higher again to attract new sellers. That's when you get a breakout and potentially a new uptrend.

That might be what's happening with Rivian.

The stock has run into resistance at $18.10 twice before. Rallies in early November and early December both topped out right around that level. Yesterday, Rivian cleared it. If the investors and traders who created that resistance have finished or pulled their orders, there's less supply standing in the way. With that obstacle removed, the stage could be set for the electric vehicle maker to push higher.

Of course, breaking resistance doesn't guarantee a rally. But it does shift the supply-demand dynamics in favor of the bulls. And in a market driven by those forces, that matters.